Fix code in make_sortkey() that only worked by chance

(assert added by MySQL verified that strnxfrm can only *increase*
the string length if from == to, and the latter is a random
decision made by individual items and String::realloc).
  
Fix it by avoiding the memcpy in the first place.

sql/filesort.cc

@@ -985,8 +985,8 @@ static void make_sortkey(register Sort_param *param,
 
         if (maybe_null)
           *to++=1;
-        /* All item->str() to use some extra byte for end null.. */
-        String tmp((char*) to,sort_field->length+4,cs);
+        char *tmp_buffer= param->tmp_buffer ? param->tmp_buffer : (char*)to;
+        String tmp(tmp_buffer, param->sort_length, cs);
         String *res= item->str_result(&tmp);
         if (!res)
         {
@@ -1011,19 +1011,11 @@ static void make_sortkey(register Sort_param *param,
         length= res->length();
         if (sort_field->need_strxnfrm)
         {
-          char *from=(char*) res->ptr();
           uint tmp_length __attribute__((unused));
-          if ((uchar*) from == to)
-          {
-            DBUG_ASSERT(sort_field->length >= length);
-            set_if_smaller(length,sort_field->length);
-            memcpy(param->tmp_buffer,from,length);
-            from=param->tmp_buffer;
-          }
           tmp_length= cs->coll->strnxfrm(cs, to, sort_field->length,
                                          item->max_char_length() * 
                                          cs->strxfrm_multiply,
-                                         (uchar*) from, length,
+                                         (uchar*) res->ptr(), length,
                                          MY_STRXFRM_PAD_WITH_SPACE |
                                          MY_STRXFRM_PAD_TO_MAXLEN);
           DBUG_ASSERT(tmp_length == sort_field->length);
@@ -1046,6 +1038,7 @@ static void make_sortkey(register Sort_param *param,
             store_length(to + sort_field_length, length,
                          sort_field->suffix_length);
           }
+          /* apply cs->sort_order for case-insensitive comparison if needed */
           my_strnxfrm(cs,(uchar*)to,length,(const uchar*)res->ptr(),length);
           cs->cset->fill(cs, (char *)to+length,diff,fill_char);
         }