glsl: Make sure not to dereference NULL
In this bit of code point_five can be NULL if the expression is not a constant. This fixes it to match the pattern of the rest of the chunk of code so that it checks for NULLs. Cc: Matt Turner <mattst88@gmail.com> Cc: "10.6" <mesa-stable@lists.freedesktop.org> Reviewed-by:Matt Turner <mattst88@gmail.com> (cherry picked from commit 86a3557d)