Change to LONGLONG_BUFFER_SIZE usage to avoid extra mallocs

This change is needed in 10.5 to avoid extra malloc calls in val_str().
In 10.6 it's not needed anymore but the extra +1 byte doesn't harm
that much.

sql/field.cc

@@ -1085,7 +1085,7 @@ Field_longstr::make_packed_sort_key_part(uchar *buff,
 uchar*
 Field_longstr::pack_sort_string(uchar *to, const SORT_FIELD_ATTR *sort_field)
 {
-  StringBuffer<LONGLONG_BUFFER_SIZE> buf;
+  StringBuffer<LONGLONG_BUFFER_SIZE+1> buf;
   val_str(&buf, &buf);
   return to + sort_field->pack_sort_string(to, &buf, field_charset());
 }

sql/item.cc

@@ -3634,7 +3634,7 @@ String *Item_int::val_str(String *str)
 
 void Item_int::print(String *str, enum_query_type query_type)
 {
-  StringBuffer<LONGLONG_BUFFER_SIZE> buf;
+  StringBuffer<LONGLONG_BUFFER_SIZE+1> buf;
   // my_charset_bin is good enough for numbers
   buf.set_int(value, unsigned_flag, &my_charset_bin);
   str->append(buf);