Fix avfilter_parse_graph() invalid graph description detection.

See the thread:
"[FFmpeg-devel] [PATCH] Fix avfilter-parse-graph()".

Originally committed as revision 17222 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/graphparser.c

@@ -376,7 +376,7 @@ int avfilter_parse_graph(AVFilterGraph *graph, const char *filters,
         index++;
     } while(chr == ',' || chr == ';');
 
-    if (*filters) {
+    if (chr) {
         av_log(log_ctx, AV_LOG_ERROR,
                "Unable to parse graph description substring: \"%s\"\n",
                filters - 1);