fuzz-journal-remote: do not assert on resource conditions

fuzz-journal-remote: do not assert on resource conditions

We have a number of issues where oss-fuzz reports input-independent crashes of
fuzz-journal-remote. Instead of asserting that stuff that allocated fds and
memory never fails, let's instead just return an error.

https://oss-fuzz.com/testcase-detail/4791099424112640
https://oss-fuzz.com/testcase-detail/4531912477638656

I don't see any docs as to whether LLVMFuzzerTestOneInput() is allowed to
return non-zero. Propagating the error code is easiest, so let's just do that.
If it turns out that this causes oss-fuzz to still report a failure, we can
suppress that later.