main: assert(texImage) in ARB_DSA texture cube map functions.

main: assert(texImage) in ARB_DSA texture cube map functions.

ARB_direct_state_access functions that deal with texture cube
maps need to make sure that texture images are not NULL before operating on
them. In the following cases, the error check functions already throw an
error if texImage == NULL, so an assert can be raised instead.

v2: Review from Anuj Phogat
   - Replace redundant "if (!texImage) return;" statements with
     assert(texImage)

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>