Only free '*ic_ptr' when a caller has pre-allocated a context and passed it in

(wherein av_open_input_file assumes memory ownership).
Patch by Art Clarke a<surname> xuggle com

Originally committed as revision 19436 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -499,7 +499,9 @@ int av_open_input_file(AVFormatContext **ic_ptr, const char *filename,
     av_freep(&pd->buf);
     if (pb)
         url_fclose(pb);
-    av_freep(ic_ptr);
+    if (ap && ap->prealloced_context)
+        av_free(*ic_ptr);
+    *ic_ptr = NULL;
     return err;
 
 }