Optimize stack usage of ha_federated::update_row().

sql/ha_federated.cc:
  We only need one string buffer for fields in ::update_row()

sql/ha_federated.cc

@@ -1817,19 +1817,13 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
   /* 
     buffers for following strings
   */
-  char old_field_value_buffer[STRING_BUFFER_USUAL_SIZE];
-  char new_field_value_buffer[STRING_BUFFER_USUAL_SIZE];
+  char field_value_buffer[STRING_BUFFER_USUAL_SIZE];
   char update_buffer[FEDERATED_QUERY_BUFFER_SIZE];
   char where_buffer[FEDERATED_QUERY_BUFFER_SIZE];
 
-  /* stores the value to be replaced of the field were are updating */
-  String old_field_value(old_field_value_buffer,
-                         sizeof(old_field_value_buffer),
-                         &my_charset_bin);
-  /* stores the new value of the field */
-  String new_field_value(new_field_value_buffer,
-                         sizeof(new_field_value_buffer),
-                         &my_charset_bin);
+  /* Work area for field values */
+  String field_value(field_value_buffer, sizeof(field_value_buffer),
+                     &my_charset_bin);
   /* stores the update query */
   String update_string(update_buffer,
                        sizeof(update_buffer),
@@ -1842,8 +1836,7 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
   /* 
     set string lengths to 0 to avoid misc chars in string
   */
-  old_field_value.length(0);
-  new_field_value.length(0);
+  field_value.length(0);
   update_string.length(0);
   where_string.length(0);
 
@@ -1874,10 +1867,10 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
     else
     {
       /* otherwise = */
-      (*field)->val_str(&new_field_value);
-      (*field)->quote_data(&new_field_value);
-      update_string.append(new_field_value);
-      new_field_value.length(0);
+      (*field)->val_str(&field_value);
+      (*field)->quote_data(&field_value);
+      update_string.append(field_value);
+      field_value.length(0);
     }
 
     if (field_in_record_is_null(table, *field, (char*) old_data))
@@ -1885,11 +1878,11 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
     else
     {
       where_string.append(FEDERATED_EQ);
-      (*field)->val_str(&old_field_value,
+      (*field)->val_str(&field_value,
                         (char*) (old_data + (*field)->offset()));
-      (*field)->quote_data(&old_field_value);
-      where_string.append(old_field_value);
-      old_field_value.length(0);
+      (*field)->quote_data(&field_value);
+      where_string.append(field_value);
+      field_value.length(0);
     }
 
     /*