glsl: avoid accessing invalid memory after get_variable_being_redeclared()

glsl: avoid accessing invalid memory after get_variable_being_redeclared()

After get_variable_being_redeclared() has been called, it is no longer
safe to access the original variable pointer, since its memory might have
been freed.

Since callers of this function should only be accessing the variable pointer
returned by the function, avoid potential bugs by re-assigning the
original variable pointer to the result of the function call,
making it impossible for the remaining code to access an invalid variable
pointer.

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