encoder: fix audio only initialization

MediaRecorder only calls the MediaEncoder with an AVFrame*, not with our
VideoFrame/AudioFrame wrappers, so the audio stream was never
initialized.

Gitlab: #95
Change-Id: Ie9c85eb9334444f2b58a4355e8f6db9d2a14610f
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>

src/media/media_encoder.cpp

@@ -469,9 +469,13 @@ MediaEncoder::encode(AVFrame* frame, int streamIdx)
     if (!initialized_ && frame) {
         // Initialize on first video frame, or first audio frame if no video stream
         bool isVideo = (frame->width > 0 && frame->height > 0);
-        if (isVideo or not videoOpts_.isValid()) {
+        if (isVideo and videoOpts_.isValid()) {
+            // Has video stream, so init with video frame
             streamIdx = initStream(videoCodec_, frame->hw_frames_ctx);
             startIO();
+        } else if (!isVideo and !videoOpts_.isValid()) {
+            // Only audio, for MediaRecorder, which doesn't use encodeAudio
+            startIO();
         } else {
             return 0;
         }