transcode: Fix filename usage for output format detection

If movs faststart option is set, the muxer will try to reopen the
output file using the path passed to avformat_alloc_output_context2
instead of the one passed to avio_open.

transcode.c

@@ -267,8 +267,9 @@ int main(int argc, char *argv[])
 	for (p = jenter(jlookup(input, "streams")); p; p = jnext(p))
 		setup_input_stream(p, demux, decs, fg, srcs, &inpads);
 
-	err = avformat_alloc_output_context2(&mux, 0,
-			jstr(jlookup(output, "format"), 0), outpath);
+	err = avformat_alloc_output_context2(&mux,
+			av_guess_format(jstr(jlookup(output, "format"), 0), outpath, 0), 0,
+			tmppath);
 	if (err < 0)
 		job_failed("Error allocating muxer context: %s", av_err2str(err));
 	parse_dict(&mux->metadata, jlookup(output, "metadata"));