virDomainDefParseXML: set the argument of virBitmapFree to NULL after calling virBitmapFree

After freeing the bitmap pointer, it must set the pointer to NULL.
This will avoid any other use of the freed memory of the bitmap pointer.

https://bugzilla.redhat.com/show_bug.cgi?id=1006710Signed-off-by: NLiuji (Jeremy) <jeremy.liu@huawei.com>
(cherry picked from commit ef5d51d4)

Conflicts:
	src/conf/domain_conf.c

src/conf/domain_conf.c

@@ -8880,8 +8880,10 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
                     }
 
                     /* Ignore 'nodeset' if 'placement' is 'auto' finally */
-                    if (placement_mode == VIR_DOMAIN_NUMATUNE_MEM_PLACEMENT_MODE_AUTO)
+                    if (placement_mode == VIR_DOMAIN_NUMATUNE_MEM_PLACEMENT_MODE_AUTO) {
                         virBitmapFree(def->numatune.memory.nodemask);
+                        def->numatune.memory.nodemask = NULL;
+                    }
 
                     /* Copy 'placement' of <numatune> to <vcpu> if its 'placement'
                      * is not specified and 'placement' of <numatune> is specified.