Fix var src_extra_specs error

This error was discovered by tool coverity. If we don't
initialize this var src_extra_specs, line 225 may sometimes
raise an error.

Change-Id: I992b56b64d56f35c8355b22707c3db5112964b31

watcher/decision_engine/strategy/strategies/storage_capacity_balance.py

@@ -207,6 +207,7 @@ class StorageCapacityBalance(base.WorkloadStabilizationBaseStrategy):
 
     def check_pool_type(self, volume, dest_pool):
         target_type = None
+        src_extra_specs = {}
         # check type feature
         if not volume.volume_type:
             return target_type