BUG#31277 - myisamchk --unpack corrupts a table

Another try to fix a compiler warning on win64.

myisam/mi_packrec.c

@@ -563,7 +563,7 @@ static void fill_quick_table(uint16 *table, uint bits, uint max_bits,
   */
   value|= (max_bits - bits) << 8 | IS_CHAR;
 
-  for (end= table + (uint) (((uint) 1 << bits)); table < end; table++)
+  for (end= table + ((my_ptrdiff_t) 1 << bits); table < end; table++)
   {
     *table= (uint16) value;
   }