audio_frame_resizer: prevent bad function calls

Change-Id: Ic8dd7dce8ca618a2e0a052db5afdf6eee80e06fa
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>

src/media/audio/audio_frame_resizer.cpp

@@ -98,6 +98,7 @@ AudioFrameResizer::enqueue(std::shared_ptr<AudioFrame>&& frame)
     auto nb_samples = samples();
     if (nb_samples == 0 && f->nb_samples == frameSize_) {
         nextOutputPts_ = frame->pointer()->pts + frameSize_;
+        if (cb_)
             cb_(std::move(frame));
         return; // return if frame was just passed through
     }