avcodec/parser: assert that there is a past buffer if theres a reference into the past

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/parser.c

@@ -285,6 +285,8 @@ int ff_combine_frame(ParseContext *pc, int next,
         return -1;
     }
 
+    av_assert0(next >= 0 || pc->buffer);
+
     *buf_size          =
     pc->overread_index = pc->index + next;