maps007 test: fix for clang

CLang doesn't like explicit NULL pointer dereference:
> maps007.c:176:2: error: indirection of non-volatile null pointer will be deleted, not trap [-Werror,-Wnull-dereference]
>         *((int *) 0) = 0;
>         ^~~~~~~~~~~~
> maps007.c:176:2: note: consider using __builtin_trap() or qualifying pointer with 'volatile'
>

Apparently, we need to add volatile to make it happy.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/transition/maps007.c

@@ -173,6 +173,6 @@ int main(int argc, char **argv)
 	return 0;
 err:
 	kill(child, SIGSEGV);
-	*((int *) 0) = 0;
+	*((volatile int *) 0) = 0;
 	return 1;
 }