Initialize mGlxPictures pointer to NULL.

Initialize mGlxPictures pointer to NULL. Otherwise
this will contain garbage value and PutSurface() will be
called on invalid pixmap, causing a segfault. This issue
was seen during VP8 playback on 64-bit bayleybay.

Signed-off-by: Sameer Kibey <sameer.kibey@intel.com>