server: fix checking disk offering access for snapshot volume (#3791)

Fixes #3783
As reported in the issue, creating volumes from pure snapshot fails with NPE. This is due to order of calls where disk offering access is checked before checking disk offering value. This PR fixes the same.

Signed-off-by: Abhishek Kumar <abhishek.mrt22@gmail.com>

Author: shwstppr

server/src/main/java/com/cloud/storage/VolumeApiServiceImpl.java

@@ -673,8 +673,6 @@ public VolumeVO allocVolume(CreateVolumeCmd cmd) throws ResourceAllocationExcept
                 zoneId = snapshotCheck.getDataCenterId();
             }
 
-            _configMgr.checkDiskOfferingAccess(null, diskOffering, _dcDao.findById(zoneId));
-
             if (diskOffering == null) { // Pure snapshot is being used to create volume.
                 diskOfferingId = snapshotCheck.getDiskOfferingId();
                 diskOffering = _diskOfferingDao.findById(diskOfferingId);
@@ -689,6 +687,8 @@ public VolumeVO allocVolume(CreateVolumeCmd cmd) throws ResourceAllocationExcept
                 }
             }
 
+            _configMgr.checkDiskOfferingAccess(null, diskOffering, _dcDao.findById(zoneId));
+
             // check snapshot permissions
             _accountMgr.checkAccess(caller, null, true, snapshotCheck);