Fix for bug #8651 (cast(sum(distinct) as decimal) crashes the server)

sql/item_sum.cc:
  curr_dec_buff should be set to 0 here

sql/item_sum.cc

@@ -656,7 +656,8 @@ my_decimal *Item_sum_sum_distinct::val_decimal(my_decimal *fake)
   else
   {
     double real= val_real();
-    double2my_decimal(E_DEC_FATAL_ERROR, real, dec_buffs + curr_dec_buff);
+    curr_dec_buff= 0;
+    double2my_decimal(E_DEC_FATAL_ERROR, real, dec_buffs);
   }
   return(dec_buffs + curr_dec_buff);
 }