My previous patch was not good at all.

This one is better, but should not be final.

Better fix is to make some changes in ha_myisammrg::index_read

This patch also reverts my previous one ...

myisammrg/myrg_rprev.c

@@ -25,6 +25,9 @@ int myrg_rprev(MYRG_INFO *info, byte *buf, int inx)
   int err;
   MI_INFO *mi;
 
+  if (!info || !info->current_table)
+    return HA_ERR_UNSUPPORTED;
+
   /* at first, do rprev for the table found before */
   if ((err=mi_rprev(info->current_table->table,NULL,inx)))
   {

sql/opt_sum.cc

@@ -152,12 +152,11 @@ int opt_sum_query(TABLE_LIST *tables, List<Item> &all_fields,COND *conds)
 	    error=table->file->index_last(table->record[0]) !=0;
 	  else
 	  {
-	    error= table->file->index_read(table->record[0], key_buff,
-					   ref.key_length,
-					   HA_READ_AFTER_KEY);
-	    if (!error)
-	      error=table->file->index_prev(table->record[0]) ||
-		key_cmp(table,key_buff,ref.key,ref.key_length);
+	    (void)table->file->index_read(table->record[0], key_buff,
+					  ref.key_length,
+					  HA_READ_AFTER_KEY);
+	    error=table->file->index_prev(table->record[0]) ||
+	      key_cmp(table,key_buff,ref.key,ref.key_length);
 	  }
 	  if (table->key_read)
 	  {