fix `mockBlock` for current slot (#2915)

In #2867 a regression was introduced that broke `mockBlock` when used to
compute a block for the current state. This patch fixes the regression.

tests/mocking/mock_blocks.nim

@@ -52,9 +52,11 @@ proc mockBlock*(
   ## TODO don't do this gradual construction, for exception safety
   ## Mock a BeaconBlock for the specific slot
 
-  var cache = StateCache()
+  var 
+    cache = StateCache()
+    tmpState = assignClone(state)
+  if getStateField(state, slot) != slot:
     var rewards = RewardInfo()
-  var tmpState = assignClone(state)
     doAssert process_slots(cfg, tmpState[], slot, cache, rewards, flags = {})
   
   var previous_block_header = getStateField(tmpState[], latest_block_header)