Apply the same logic used for subtitle stream selection to audio and video

stream selection.

Make the counting starts from 0, and disable the corresponding playbck
when the selected stream number is negative.

See the thread:
" [FFmpeg-devel] [PATCH] Add documentation for -ast, -vst, -sst".

Originally committed as revision 17693 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffplay.c

@@ -1974,11 +1974,11 @@ static int decode_thread(void *arg)
         ic->streams[i]->discard = AVDISCARD_ALL;
         switch(enc->codec_type) {
         case CODEC_TYPE_AUDIO:
-            if ((audio_index < 0 || wanted_audio_stream-- > 0) && !audio_disable)
+            if (wanted_audio_stream-- >= 0 && !audio_disable)
                 audio_index = i;
             break;
         case CODEC_TYPE_VIDEO:
-            if ((video_index < 0 || wanted_video_stream-- > 0) && !video_disable)
+            if (wanted_video_stream-- >= 0 && !video_disable)
                 video_index = i;
             break;
         case CODEC_TYPE_SUBTITLE: