Return early in yaml_emitter_close

If yaml_emitter_dump failed, the emitter might be in a broken
state and have leftover states/events.
yaml_emitter_close calls yaml_emitter_emit which will
try to emit those events, so we should return early.

The recommended thing to do for library users is to abort
when yaml_emitter_dump fails, but it's easy to add a check
here as well.

src/dumper.c

@@ -92,6 +92,7 @@ yaml_emitter_close(yaml_emitter_t *emitter)
     assert(emitter->opened);    /* Emitter should be opened. */
 
     if (emitter->closed) return 1;
+    if (emitter->error) return 0; /* Emitter might have leftover events */
 
     STREAM_END_EVENT_INIT(event, mark, mark);