BUG#17521 alter database crashes slave

Problem was a null pointer
Fixed by passing correct database name to slave filtering mechanism
Added test.

mysql-test/r/rpl_alter_db.result

+stop slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+reset master;
+reset slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+start slave;
+use mysql;
+alter database collate latin1_bin;

mysql-test/t/rpl_alter_db.test

+source include/master-slave.inc;
+connection master;
+use mysql; # to be different from initial `test' db of mysqltest client
+alter database collate latin1_bin;
+save_master_pos;
+
+connection slave;
+sync_with_master;
+
+

sql/sql_parse.cc

@@ -3715,7 +3715,7 @@ mysql_execute_command(THD *thd)
     }
     if (!strip_sp(db) || check_db_name(db))
     {
-      my_error(ER_WRONG_DB_NAME, MYF(0), lex->name);
+      my_error(ER_WRONG_DB_NAME, MYF(0), db);
       break;
     }
     /*
@@ -3727,8 +3727,8 @@ mysql_execute_command(THD *thd)
     */
 #ifdef HAVE_REPLICATION
     if (thd->slave_thread &&
-	(!rpl_filter->db_ok(lex->name) ||
-	 !rpl_filter->db_ok_with_wild_table(lex->name)))
+	(!rpl_filter->db_ok(db) ||
+	 !rpl_filter->db_ok_with_wild_table(db)))
     {
       my_message(ER_SLAVE_IGNORED_TABLE, ER(ER_SLAVE_IGNORED_TABLE), MYF(0));
       break;