h264: reset ret to avoid propagating minor failures

Unbreak 772d150a6e.

CC: libav-stable@libav.org
Bug-Id: 750 / 905753
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

libavcodec/h264.c

@@ -1679,6 +1679,7 @@ again:
             goto end;
     }
 
+    ret = 0;
 end:
     /* clean up */
     if (h->cur_pic_ptr && !h->droppable) {