fixed NPE

This closes #70

(cherry picked from commit 98b416b)
Signed-off-by: Rohit Yadav <[email protected]>

Author: Anthony Xu

server/src/com/cloud/resource/ResourceManagerImpl.java

@@ -2080,7 +2080,7 @@ private boolean doCancelMaintenance(long hostId) {
         _haMgr.cancelScheduledMigrations(host);
         List<VMInstanceVO> vms = _haMgr.findTakenMigrationWork();
         for (VMInstanceVO vm : vms) {
-            if (vm.getHostId() != null && vm.getHostId() == hostId) {
+            if (vm != null && vm.getHostId() != null && vm.getHostId() == hostId) {
                 s_logger.info("Unable to cancel migration because the vm is being migrated: " + vm);
                 return false;
             }