journal: properly handle an unexpectedly missing field

journal: properly handle an unexpectedly missing field

parse_field() checks if the field has the expected format, and returns
0 if it doesn't. In that case, value and size are not
set. Nevertheless, we would try to continue, and hit an assert in
safe_atou64. This case shouldn't happen, unless sd_j_get_data is borked,
so cleanly assert that we got the expected field.

Also, oom is the only way that parse_field can fail, which we log
already. Instead of outputting a debug statement and carrying on,
treat oom as fatal.