qemu: reorder checks for safety

Detected by Coverity.  All existing callers happen to be in
range, so this isn't too serious.

* src/qemu/qemu_cgroup.c (qemuCgroupControllerActive): Check
bounds before dereference.

src/qemu/qemu_cgroup.c

@@ -48,10 +48,10 @@ bool qemuCgroupControllerActive(struct qemud_driver *driver,
 {
     if (driver->cgroup == NULL)
         return false;
-    if (!virCgroupMounted(driver->cgroup, controller))
-        return false;
     if (controller < 0 || controller >= VIR_CGROUP_CONTROLLER_LAST)
         return false;
+    if (!virCgroupMounted(driver->cgroup, controller))
+        return false;
     if (driver->cgroupControllers & (1 << controller))
         return true;
     return false;