core: set all log fds to -1 when freezing (#3314)

Fixes:
-bash-4.3# echo core >/proc/sys/kernel/core_pattern
-bash-4.3# kill -ABRT 1
-bash-4.3# kill -ABRT 1
[   61.373922] systemd[1]: segfault at 7fff1d0a8f48 ip 00007fc9ca91b1c3 sp 00007fff1d0a8f50 error 6 in libc-2.23.so[7fc9ca8ce000+1c0000]
[   61.768017] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000008b
[   61.768017]
...

Recursive ABRT and segfault:
PID 1 - core
TID 1:
...
 #153905 0x00005575fc3f829d log_dispatch
 #153906 0x00005575fc3f8aa3 log_assert
 #153907 0x00005575fc3f8ae9 log_assert_failed
 #153908 0x00005575fc3e7eb1 safe_close
 #153909 0x00005575fc3f6d5e log_close_journal
 #153910 0x00005575fc3f829d log_dispatch
 #153911 0x00005575fc3f85a1 log_internalv
 #153912 0x00005575fc3f86a1 log_internal
 #153913 0x00005575fc31c4c1 crash
 #153914 0x00007fb26f2cf3d0 __restore_rt
 #153915 0x00007fb26f2ced00 pause
 #153916 0x00005575fc403944 freeze
 #153917 0x00005575fc31bf7b freeze_or_reboot
...

src/basic/process-util.c

@@ -667,6 +667,8 @@ bool is_main_thread(void) {
 
 noreturn void freeze(void) {
 
+        log_close();
+
         /* Make sure nobody waits for us on a socket anymore */
         close_all_fds(NULL, 0);