x86-64, NUMA: Don't call __pa() with invalid address in numa_reset_distance()

Do not call __pa(numa_distance) if it was not allocated before.
Calling with invalid address triggers VIRTUAL_BUG_ON() in
__phys_addr() if CONFIG_DEBUG_VIRTUAL.

Also reported by Ingo.

 http://thread.gmane.org/gmane.linux.kernel/1101306/focus=1101785

- v2: Change to check existing path as tj requested.
- tj: Description update.
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/numa_64.c

@@ -371,11 +371,13 @@ static void __init numa_reset_distance(void)
 {
 	size_t size;
 
+	if (numa_distance_cnt) {
 		size = numa_distance_cnt * sizeof(numa_distance[0]);
 		memblock_x86_free_range(__pa(numa_distance),
 					__pa(numa_distance) + size);
-	numa_distance = NULL;
 		numa_distance_cnt = 0;
+	}
+	numa_distance = NULL;
 }
 
 /*