glcpp: Fix segfault when validating macro redefinitions

In _token_list_equal_ignoring_space(token_list_t*, token_list_t*), add
a guard that prevents dereferncing a null token list.

This fixes test src/glsl/glcpp/tests/092-redefine-macro-error-2.c and
Bugzilla #32695.

src/glsl/glcpp/glcpp-parse.y

@@ -824,11 +824,32 @@ _token_list_trim_trailing_space (token_list_t *list)
 	}
 }
 
+int
+_token_list_is_empty_ignoring_space (token_list_t *l)
+{
+	token_node_t *n;
+
+	if (l == NULL)
+		return 1;
+
+	n = l->head;
+	while (n != NULL && n->token->type == SPACE)
+		n = n->next;
+
+	return n == NULL;
+}
+
 int
 _token_list_equal_ignoring_space (token_list_t *a, token_list_t *b)
 {
 	token_node_t *node_a, *node_b;
 
+	if (a == NULL || b == NULL) {
+		int a_empty = _token_list_is_empty_ignoring_space(a);
+		int b_empty = _token_list_is_empty_ignoring_space(b);
+		return a_empty == b_empty;
+	}
+
 	node_a = a->head;
 	node_b = b->head;