Do not probe when the format is known.

Patch by Jean-Daniel Dupas devlists shadowlab org

Originally committed as revision 22870 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -559,7 +559,7 @@ int av_open_input_file(AVFormatContext **ic_ptr, const char *filename,
         if (buf_size > 0) {
             url_setbufsize(pb, buf_size);
         }
-        if ((err = ff_probe_input_buffer(&pb, &fmt, filename, logctx, 0, logctx ? (*ic_ptr)->probesize : 0)) < 0) {
+        if (!fmt && (err = ff_probe_input_buffer(&pb, &fmt, filename, logctx, 0, logctx ? (*ic_ptr)->probesize : 0)) < 0) {
             goto fail;
         }
     }