refs #5510 get varchar expansion with prefix keys working in mysql 5.6.7

git-svn-id: file:///svn/mysql/tokudb-engine/tokudb-engine@48516 c7de825b-a66e-492c-adef-691d508d4ae1

storage/tokudb/ha_tokudb_alter_56.cc

@@ -709,7 +709,7 @@ static bool
 field_in_key(KEY *key, Field *field) {
     for (uint i = 0; i < key->key_parts; i++) {
         KEY_PART_INFO *key_part = &key->key_part[i];
-        if (key_part->field == field)
+        if (strcmp(key_part->field->field_name, field->field_name) == 0)
             return true;
     }
     return false;