avcodec/flac_parser: Update nb_headers_buffered

Fixes infinite loop
Fixes: fuzz.flac
Found-by: Frank Liberato <liberato@google.com>
Reviewed-by: Frank Liberato <liberato@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/flac_parser.c

@@ -586,10 +586,12 @@ static int flac_parse(AVCodecParserContext *s, AVCodecContext *avctx,
             temp = curr->next;
             av_freep(&curr->link_penalty);
             av_free(curr);
+            fpc->nb_headers_buffered--;
         }
         fpc->headers = fpc->best_header->next;
         av_freep(&fpc->best_header->link_penalty);
         av_freep(&fpc->best_header);
+        fpc->nb_headers_buffered--;
     }
 
     /* Find and score new headers.                                     */