ast_expr2: Fix off-nominal memory leak.

Thanks to ibercom for pointing out a memory leak that was missed
in the earlier patch for the issue.

ASTERISK-26119
Reported by: Alexei Gradinari

Change-Id: I9a151f5c4725d97fb82a9e938bc73dc659532b71

main/ast_expr2.y

@@ -1666,6 +1666,8 @@ op_tildetilde (struct val *a, struct val *b)
 	vs = malloc(strlen(a->u.s)+strlen(b->u.s)+1);
 	if (vs == NULL) {
 		ast_log(LOG_WARNING, "malloc() failed\n");
+		free_value(a);
+		free_value(b);
 		return NULL;
 	}