fixed coredump in SET PASSWORD in slave thread.

mysql-test/t/rpl000001.test

@@ -5,6 +5,8 @@ drop table if exists t1;
 create table t1 (word char(20) not null);
 load data infile '../../std_data/words.dat' into table t1;
 drop table if exists foo;
+set password = password('foo');
+set password = password('');
 create table foo(n int);
 insert into foo values(1),(2);
 save_master_pos;

sql/sql_acl.cc

@@ -781,8 +781,8 @@ bool change_password(THD *thd, const char *host, const char *user,
   length=(uint) strlen(new_password);
   new_password[length & 16]=0;
 
-  if (!thd || strcmp(thd->user,user) ||
-      my_strcasecmp(host,thd->host ? thd->host : thd->ip))
+  if (!thd || (!thd->slave_thread && ( strcmp(thd->user,user) ||
+	       my_strcasecmp(host,thd->host ? thd->host : thd->ip))))
   {
     if (check_access(thd, UPDATE_ACL, "mysql",0,1))
       return 1;