Do not use AVFMT_FLAG_NONBLOCK for the output file.

Allows to use output device alsa.

ffmpeg.c

@@ -3835,7 +3835,6 @@ static void opt_output_file(const char *filename)
     oc->preload= (int)(mux_preload*AV_TIME_BASE);
     oc->max_delay= (int)(mux_max_delay*AV_TIME_BASE);
     oc->loop_output = loop_output;
-    oc->flags |= AVFMT_FLAG_NONBLOCK;
 
     set_context_opts(oc, avformat_opts, AV_OPT_FLAG_ENCODING_PARAM, NULL);