check of mfxExtCodingOption2 for AVC only

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6112 b64f7644-9d1e-0410-96f1-a4d463321fa5
libhb: fix uninitialized variable

since a malloc depends on the value of this variable, it's rather important
that it be initialized ;)


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6113 b64f7644-9d1e-0410-96f1-a4d463321fa5