network: Fix crash when SendOption= is invalid

```
p11-kit-0.23.20-1.fc32.x86_64 pam-1.3.1-26.fc33.x86_64 xz-libs-5.2.5-1.fc33.x86_64 zlib-1.2.11-21.fc32.x86_64
(gdb) bt
    lvalue=0x560e10 "SendOption", ltype=2, rvalue=0x560e1b "11:string", data=0x561e20, userdata=0x561cd0) at ../src/network/networkd-dhcp-common.c:580
    table=0x4392e0 <network_network_gperf_lookup>, section=0x560ef0 "DHCPv4", section_line=14, lvalue=0x560e10 "SendOption", rvalue=0x560e1b "11:string", flags=CONFIG_PARSE_WARN,
    userdata=0x561cd0) at ../src/shared/conf-parser.c:132
    lookup=0x7ffff7d2f76d <config_item_perf_lookup>, table=0x4392e0 <network_network_gperf_lookup>, flags=CONFIG_PARSE_WARN, section=0x7fffffffc9f8, section_line=0x7fffffffc9a0,
    section_ignored=0x7fffffffc99d, l=0x560e10 "SendOption", userdata=0x561cd0) at ../src/shared/conf-parser.c:270
    lookup=0x7ffff7d2f76d <config_item_perf_lookup>, table=0x4392e0 <network_network_gperf_lookup>, flags=CONFIG_PARSE_WARN, userdata=0x561cd0) at ../src/shared/conf-parser.c:395
    lookup=0x7ffff7d2f76d <config_item_perf_lookup>, table=0x4392e0 <network_network_gperf_lookup>, flags=CONFIG_PARSE_WARN, userdata=0x561cd0) at ../src/shared/conf-parser.c:452
    dropin_dirname=0x7fffffffcbd0 "veth99.network.d", sections=0x4f3a18 "Match", lookup=0x7ffff7d2f76d <config_item_perf_lookup>, table=0x4392e0 <network_network_gperf_lookup>,
    flags=CONFIG_PARSE_WARN, userdata=0x561cd0) at ../src/shared/conf-parser.c:511
(gdb) q
A debugging session is active.

	Inferior 1 [process 118718] will be killed.
```

```
$ printf '[DHCPv4]\nSendOption=1:uint8' >crash

$ ./out/fuzz-network-parser ./crash
INFO: Seed: 1158717610
INFO: Loaded 2 modules   (199728 inline 8-bit counters): 136668 [0x7faf3e91a930, 0x7faf3e93bf0c), 63060 [0xadf190, 0xaee7e4),
INFO: Loaded 2 PC tables (199728 PCs): 136668 [0x7faf3e93bf10,0x7faf3eb51cd0), 63060 [0xaee7e8,0xbe4d28),
./out/fuzz-network-parser: Running 1 inputs 1 time(s) each.
Running: ./crash
Assertion 's' failed at src/basic/parse-util.c:458, function int safe_atou8(const char *, uint8_t *)(). Aborting.
==5588== ERROR: libFuzzer: deadly signal
    #0 0x51811e in __sanitizer_print_stack_trace (/home/vagrant/systemd/out/fuzz-network-parser+0x51811e)
    #1 0x46b921 in fuzzer::PrintStackTrace() (/home/vagrant/systemd/out/fuzz-network-parser+0x46b921)
    #2 0x44ded6 in fuzzer::Fuzzer::CrashCallback() (.part.0) (/home/vagrant/systemd/out/fuzz-network-parser+0x44ded6)
    #3 0x44df9d in fuzzer::Fuzzer::StaticCrashSignalCallback() (/home/vagrant/systemd/out/fuzz-network-parser+0x44df9d)
    #4 0x7faf3d6d7b1f  (/lib64/libpthread.so.0+0x14b1f)
    #5 0x7faf3d3c2624 in raise (/lib64/libc.so.6+0x3c624)
    #6 0x7faf3d3ab8d8 in abort (/lib64/libc.so.6+0x258d8)
    #7 0x7faf3e12593a in log_assert_failed_realm /home/vagrant/systemd/build/../src/basic/log.c:819:9
    #8 0x7faf3e140ce1 in safe_atou8 /home/vagrant/systemd/build/../src/basic/parse-util.c:458:9
    #9 0x68089c in config_parse_dhcp_send_option /home/vagrant/systemd/build/../src/network/networkd-dhcp-common.c:517:21
    #10 0x7faf3debed4e in next_assignment /home/vagrant/systemd/build/../src/shared/conf-parser.c:132:32
    #11 0x7faf3deb7783 in parse_line /home/vagrant/systemd/build/../src/shared/conf-parser.c:270:16
    #12 0x7faf3deb606c in config_parse /home/vagrant/systemd/build/../src/shared/conf-parser.c:395:21
    #13 0x7faf3deb85ee in config_parse_many_files /home/vagrant/systemd/build/../src/shared/conf-parser.c:452:21
    #14 0x7faf3deb8c57 in config_parse_many /home/vagrant/systemd/build/../src/shared/conf-parser.c:511:16
    #15 0x57c2eb in network_load_one /home/vagrant/systemd/build/../src/network/networkd-network.c:470:13
    #16 0x543490 in LLVMFuzzerTestOneInput /home/vagrant/systemd/build/../src/network/fuzz-network-parser.c:26:16
    #17 0x44e3e8 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/vagrant/systemd/out/fuzz-network-parser+0x44e3e8)
    #18 0x433505 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (/home/vagrant/systemd/out/fuzz-network-parser+0x433505)
    #19 0x43c449 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/vagrant/systemd/out/fuzz-network-parser+0x43c449)
    #20 0x42c4a6 in main (/home/vagrant/systemd/out/fuzz-network-parser+0x42c4a6)
    #21 0x7faf3d3ad1a2 in __libc_start_main (/lib64/libc.so.6+0x271a2)
    #22 0x42c4fd in _start (/home/vagrant/systemd/out/fuzz-network-parser+0x42c4fd)

NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal

```

src/network/networkd-dhcp-common.c

@@ -499,7 +499,7 @@ int config_parse_dhcp_send_option(
         r = extract_first_word(&p, &word, ":", 0);
         if (r == -ENOMEM)
                 return log_oom();
-        if (r <= 0) {
+        if (r <= 0 || isempty(p)) {
                 log_syntax(unit, LOG_ERR, filename, line, r,
                            "Invalid DHCP option, ignoring assignment: %s", rvalue);
                 return 0;