oops - forgot grabbing case for audio/video input auto detect

Originally committed as revision 134 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -1941,12 +1941,15 @@ void opt_output_file(const char *filename)
         use_video = file_format->video_codec != CODEC_ID_NONE;
         use_audio = file_format->audio_codec != CODEC_ID_NONE;
 
-        /* disable if no corresponding type found */
+        /* disable if no corresponding type found and at least one
+           input file */
+        if (nb_input_files > 0) {
             check_audio_video_inputs(&input_has_video, &input_has_audio);
             if (!input_has_video)
                 use_video = 0;
             if (!input_has_audio)
                 use_audio = 0;
+        }
 
         /* manual disable */
         if (audio_disable) {