journal: add an explicit check for uninitialized objects

Let's make dissecting of borked journal files more expressive: if we encounter
an object whose first 8 bytes are all zeroes, then let's assume the object was
simply never initialized, and say so.

Previously, this would be detected as "overly short object", which is true too
in a away, but it's a lot more helpful printing different debug options for the
case where the size is not initialized at all and where the size is initialized
to some bogus value.

No function behaviour change, only a different log messages for both cases.

src/journal/journal-file.c

@@ -765,6 +765,10 @@ int journal_file_move_to_object(JournalFile *f, ObjectType type, uint64_t offset
         o = (Object*) t;
         s = le64toh(o->object.size);
 
+        if (s == 0) {
+                log_debug("Attempt to move to uninitialized object: %" PRIu64, offset);
+                return -EBADMSG;
+        }
         if (s < sizeof(ObjectHeader)) {
                 log_debug("Attempt to move to overly short object: %" PRIu64, offset);
                 return -EBADMSG;