Change avfilter_open() signature, from:

AVFilterContext *avfilter_open(AVFilter *filter, const char *inst_name);
to:
int avfilter_open(AVFilterContext **filter_ctx, AVFilter *filter, const char *inst_name);

This way it is possible to propagate an error code telling the reason
of the failure.

Originally committed as revision 24765 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/graphparser.c

@@ -111,7 +111,7 @@ static AVFilterContext *create_filter(AVFilterGraph *ctx, int index,
         return NULL;
     }
 
-    filt_ctx = avfilter_open(filt, inst_name);
+    avfilter_open(&filt_ctx, filt, inst_name);
     if (!filt_ctx) {
         av_log(log_ctx, AV_LOG_ERROR,
                "Error creating filter '%s'\n", filt_name);