Core: Clean up here and there.

*replase postfix iterator increment\decrement with prefix in cycles
*replase strlen(*char) != 0 with *char[0] != '\0'
*replase strlen(rStr.c_str()) with rStr.length()

src/server/game/Groups/Group.cpp

@@ -1003,7 +1003,7 @@ void Group::EndRoll(Loot *pLoot)
             itr = RollId.begin();
         }
         else
-            itr++;
+            ++itr;
     }
 }