mesa: Fix assertion failure when a cube face is not present.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/texobj.c

@@ -558,7 +558,8 @@ _mesa_test_texobj_completeness( const struct gl_context *ctx,
       GLuint face;
       assert(baseImage->Width2 == baseImage->Height);
       for (face = 1; face < 6; face++) {
-         assert(t->Image[face][baseLevel]->Width2 ==
+         assert(t->Image[face][baseLevel] == NULL ||
+                t->Image[face][baseLevel]->Width2 ==
                 t->Image[face][baseLevel]->Height2);
          if (t->Image[face][baseLevel] == NULL ||
              t->Image[face][baseLevel]->Width2 != baseImage->Width2) {