Fix unitialized first_pixels for foreign textures

Fix unitialized first_pixels for foreign textures

tex->first_pixels was never set for foreign textures, leading
to a crash when the texture object is freed.

As a quick fix, simply set to NULL. A more complete fix would
require remembering if we had ever seen the first pixel uploaded,
and if not, doing a glReadPixel to get it before triggering the
mipmap update.

http://bugzilla.openedhand.com/show_bug.cgi?id=1645

Signed-off-by: Neil Roberts <neil@linux.intel.com>