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.

src/shared/logs-show.c

@@ -435,8 +435,9 @@ static int output_verbose(
 
                 r = parse_field(data, length, "_SOURCE_REALTIME_TIMESTAMP=", &value, &size);
                 if (r < 0)
-                        log_debug_errno(r, "_SOURCE_REALTIME_TIMESTAMP invalid: %m");
+                        return r;
                 else {
+                        assert(r > 0);
                         r = safe_atou64(value, &realtime);
                         if (r < 0)
                                 log_debug_errno(r, "Failed to parse realtime timestamp: %m");