virnuma: add nodeset NULL check in virNumaSetupMemoryPolicy

Introduced by commit c63ef045, when nodeset is NULL, validation will
pass in virNumaSetupMemoryPolicy, but virBitmapNextSetBit must ensure
bitmap is not NULL, otherwise that might cause a segmentation fault.
This patch fixes it.
Signed-off-by: NChen Fan <chen.fan.fnst@cn.fujitsu.com>

src/util/virnuma.c

@@ -97,6 +97,9 @@ virNumaSetupMemoryPolicy(virDomainNumatuneMemMode mode,
     size_t i;
     int maxnode = 0;
 
+    if (!nodeset)
+        return 0;
+
     if (!virNumaNodesetIsAvailable(nodeset))
         return -1;