glsl: Protect against double compiler-destroy.

DRI was doing teardown when we close the last screen, then an atexit()
was added to call it as well.

src/glsl/builtins/tools/generate_builtins.py

@@ -175,6 +175,7 @@ void
 _mesa_glsl_release_functions(void)
 {
     talloc_free(builtin_mem_ctx);
+    builtin_mem_ctx = NULL;
 }
 
 void