qemu_cgroup: initialize mem_mask to NULL

If 'virNumaGetHostNodeset()' fails then the error path will try to free
uninitialized pointer mem_mask. Introduced by commit af2a1f05.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>
(cherry picked from commit 77a9dc0b)
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/qemu/qemu_cgroup.c

@@ -789,7 +789,7 @@ qemuInitCgroup(virQEMUDriverPtr driver,
 static void
 qemuRestoreCgroupState(virDomainObjPtr vm)
 {
-    char *mem_mask;
+    char *mem_mask = NULL;
     int empty = -1;
     qemuDomainObjPrivatePtr priv = vm->privateData;
     virBitmapPtr all_nodes;