glsl: make the redeclared variable NULL if it is deleted

glsl: make the redeclared variable NULL if it is deleted

get_variable_being_redeclared() can delete the original variable
in a specific scenario. The code sets it to NULL after this so other
code in that same function doesn't try to access trashed memory after
the fact, however, the copy of that variable in the caller code
won't see any of this making it very easy to overlook.

Make the function a bit safer by taking a pointer to the original
variable so we can also make NULL the caller's pointer to the variable
if this function deletes it.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>