fuzz-journal-stream: avoid assertion failure on samples which don't fit in pipe

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11587.
We had a sample which was large enough that write(2) failed to push all the
data into the pipe, and an assert failed. The code could be changed to use
a loop, but then we'd need to interleave writes and sd_event_run (to process
the journal). I don't think the complexity is worth it — fuzzing works best
if the sample is not too huge anyway. So let's just reject samples above 64k,
and tell oss-fuzz about this limit.

src/fuzz/fuzz-journald-stream.c

@@ -15,7 +15,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         StdoutStream *stream;
         int v;
 
-        if (size == 0)
+        if (size == 0 || size > 65536)
                 return 0;
 
         if (!getenv("SYSTEMD_LOG_LEVEL"))

src/fuzz/fuzz-journald-stream.options

@@ -0,0 +1,2 @@
+[libfuzzer]
+max_len = 65536