Fix "cast to pointer from integer of different size"

Fix "cast to pointer from integer of different size"

The script generates code like:
   pixels = (const GLvoid *) (ptr_is_null != 0) ? NULL : (pc + 80);
which causes the above mentioned warning. Add parenthesis around the
whole expression to fix it.

Signed-off-by: Tomas Carnecky <tom@dbservice.com>