[rbp/omxplayer] Avoid segfault when switching fullscreen

Switching fullscreen causes a brief UnregisterAudioCallback/RegsisterAudioCallback.
If this occurs while audio thread is inside VizPacket then we segfault on dereferencing m_pCallback.

Using the m_critSection lock means the higher level test for m_pCallback will protect us.
Merge pull request #3749 from popcornmix/fullscreen_seg

[rbp/omxplayer] Avoid segfault when switching fullscreen