meta: Abort texture upload if pixels == null and no pixel unpack buffer set

meta: Abort texture upload if pixels == null and no pixel unpack buffer set

in case of glTexImage{1,2,3}D(). Texture has already been allocated
at this point and we have no data to upload. With out this patch,
with create_pbo = true, we end up creating a temporary pbo and then
uploading uninitialzed texture data.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Neil Roberts <neil@linux.intel.com>