[rbp/omxplayer] potential unguarded null pointer when video has no audio

... (e.g. stills in dvd menu)

Copy of https://github.com/xbmc/xbmc/pull/3566

@Voyager1
Feel free to edit omxplayer code to match changes to dvdplayer (I'll test if you can't) -
OMXPlayer.cpp and DVDPlayer.cpp are almost identical, so copy+paste (and maybe change dvd to omx) will usually do.