storage: Fix error path in storagePoolDefineXML

storage: Fix error path in storagePoolDefineXML

Found by inspection - after calling virStoragePoolObjAssignDef the
pool is part of the driver->pools.objs list and the failure path
for the virStoragePoolObjSaveDef will use virStoragePoolObjRemove
to remove the pool from the objs list which will unlock and free
the pool pointer (as pools->objs[i] during the loop). Since the call
doesn't clear the pool address from the callee, we need to set it
to NULL; otherwise, the virStoragePoolObjUnlock in the cleanup: code
will fail miserably.