mysql57: follow THD::db type change

I don't know whether THD::db() (LEX_CSTRING) is NULL
terminated. Generally, LEX_CSTRING isn't NULL terminated.

If THD::db() isn't NULL terminated, Mroonga may crash.

mrn_mysql_compat.h

@@ -1,6 +1,6 @@
 /* -*- c-basic-offset: 2 -*- */
 /*
-  Copyright(C) 2011-2014 Kouhei Sutou <kou@clear-code.com>
+  Copyright(C) 2011-2015 Kouhei Sutou <kou@clear-code.com>
 
   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
@@ -158,4 +158,10 @@
 #  define MRN_STRING_FREE(string) string.free();
 #endif
 
+#if MYSQL_VERSION_ID >= 50706 && !defined(MRN_MARIADB_P)
+#  define MRN_THD_DB_PATH(thd) ((thd)->db().str)
+#else
+#  define MRN_THD_DB_PATH(thd) ((thd)->db)
+#endif
+
 #endif /* MRN_MYSQL_COMPAT_H_ */

udf/mrn_udf_command.cpp

@@ -68,7 +68,7 @@ MRN_API my_bool mroonga_command_init(UDF_INIT *initid, UDF_ARGS *args,
 
   grn_ctx_init(&(info->ctx), 0);
   {
-    const char *current_db_path = current_thd->db;
+    const char *current_db_path = MRN_THD_DB_PATH(current_thd);
     const char *action;
     if (current_db_path) {
       action = "open database";