Semi-join fixes: make COST_VECT objects survive add_io(add_io_cnt=0,...

Semi-join fixes: make COST_VECT objects survive add_io(add_io_cnt=0, add_avg_cost=...) calls without
getting NaN in internal fields.

sql/handler.h

@@ -1389,12 +1389,16 @@ class COST_VECT
     cpu_cost += cost->cpu_cost;
   }
   void add_io(double add_io_cnt, double add_avg_cost)
+  {
+    /* In edge cases add_io_cnt may be zero */
+    if (add_io_cnt > 0)
     {
       double io_count_sum= io_count + add_io_cnt;
       avg_io_cost= (io_count * avg_io_cost + 
                     add_io_cnt * add_avg_cost) / io_count_sum;
       io_count= io_count_sum;
     }
+  }
 
   /*
     To be used when we go from old single value-based cost calculations to