Fix compile failure on Windows: use explicit type casts

sql/multi_range_read.cc

@@ -1714,7 +1714,7 @@ bool DsMrr_impl::get_disk_sweep_mrr_cost(uint keynr, ha_rows rows, uint flags,
       Request memory to finish the scan in one pass but do not request
       more than @@mrr_buff_size.
     */
-    *buffer_size = MY_MIN(extra_mem_overhead + rows*elem_size,
+    *buffer_size= (uint) MY_MIN(extra_mem_overhead + elem_size*(ulong)rows,
                                 MY_MAX(table->in_use->variables.mrr_buff_size,
                                        extra_mem_overhead));
   }