Fixed a warning on win64.

myisam/mi_rkey.c

@@ -62,7 +62,8 @@ int mi_rkey(MI_INFO *info, byte *buf, int inx, const byte *key, uint key_len,
 				 key_len, &last_used_keyseg);
     /* Save packed_key_length for use by the MERGE engine. */
     info->pack_key_length= pack_key_length;
-    info->last_used_keyseg= last_used_keyseg - info->s->keyinfo[inx].seg;
+    info->last_used_keyseg= (uint16) (last_used_keyseg -
+                                      info->s->keyinfo[inx].seg);
     DBUG_EXECUTE("key",_mi_print_key(DBUG_FILE, keyinfo->seg,
 				     key_buff, pack_key_length););
   }