Fixes mediaplayer crashes on OSX

Fixes the "libqavfmediaplayer.dylib 0x0000000110fa7c8c -[AVFMediaPlayerSessionObserver unloadMedia]" crash on Macs.
The problem was: writing to memory that had already been released. If not sure, one should always check if the objects exist before deleting it.

Solution tested on OSX 10.7, 10.8, 10.9

[ChangeLog][qtmultimedia][avfmediaplayersession]

Task-number: QTBUG-34213
Change-Id: Iac108711851c348e96e73542b4e71653007eeb54
Reviewed-by: Andy Nichols <andy.nichols@digia.com>