MyISAM was erroneously using max_ref_length when estimating min_pack_length.

myisam/mi_packrec.c

@@ -165,7 +165,9 @@ my_bool _mi_read_pack_info(MI_INFO *info, pbool fix_keys)
   diff_length=(int) rec_reflength - (int) share->base.rec_reflength;
   if (fix_keys)
     share->rec_reflength=rec_reflength;
-  share->base.min_block_length=share->min_pack_length+share->pack.ref_length;
+  share->base.min_block_length=share->min_pack_length+1;
+  if (share->min_pack_length > 254)
+    share->base.min_block_length+=2;
 
   if (!(share->decode_trees=(MI_DECODE_TREE*)
 	my_malloc((uint) (trees*sizeof(MI_DECODE_TREE)+