MDEV-17119 replicate_rewrite_db does not work for single chardatabase name

Fixed issue in logic.

mysql-test/suite/rpl/r/rpl_rewrt_db.result

 include/master-slave.inc
 [connection master]
+set sql_log_bin=0;
+create database y;
+set sql_log_bin=1;
 drop database if exists mysqltest1;
+drop database if exists x;
 create database mysqltest1;
+set sql_log_bin=0;
+create database x;
+set sql_log_bin=1;
 use mysqltest1;
 create table t1 (a int);
 insert into t1 values(9);
+use x;
+create table t1 (a int);
+insert into t1 values(9);
 select * from mysqltest1.t1;
 a
 9
+select * from x.t1;
+a
+9
 show databases like 'mysqltest1';
 Database (mysqltest1)
 mysqltest1
 select * from test.t1;
 a
 9
+select * from y.t1;
+a
+9
+use mysqltest1;
 drop table t1;
 drop database mysqltest1;
 drop database if exists rewrite;
@@ -209,10 +226,12 @@ SET sql_log_bin= 0;
 DROP DATABASE database_master_temp_01;
 DROP DATABASE database_master_temp_02;
 DROP DATABASE database_master_temp_03;
+DROP DATABASE x;
 SET sql_log_bin= 1;
 SET sql_log_bin= 0;
 DROP DATABASE database_slave_temp_01;
 DROP DATABASE database_slave_temp_02;
 DROP DATABASE database_slave_temp_03;
+DROP DATABASE y;
 SET sql_log_bin= 1;
 include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_rewrt_db-slave.opt

-"--replicate-rewrite-db=test->rewrite" "--replicate-rewrite-db=mysqltest1->test"  "--replicate-rewrite-db=database_master_temp_01->database_slave_temp_01" "--replicate-rewrite-db=database_master_temp_02->database_slave_temp_02" "--replicate-rewrite-db=database_master_temp_03->database_slave_temp_03"
+"--replicate-rewrite-db=test->rewrite" "--replicate-rewrite-db=mysqltest1 -> test" "--replicate-rewrite-db=x -> y" "--replicate-rewrite-db=database_master_temp_01->database_slave_temp_01" "--replicate-rewrite-db=database_master_temp_02->database_slave_temp_02" "--replicate-rewrite-db=database_master_temp_03->database_slave_temp_03"

mysql-test/suite/rpl/t/rpl_rewrt_db.test

@@ -2,20 +2,37 @@
 -- source include/have_binlog_format_mixed_or_statement.inc
 -- source include/master-slave.inc
 
+--connection slave
+set sql_log_bin=0;
+create database y;
+set sql_log_bin=1;
+
+--connection master
 --disable_warnings
 drop database if exists mysqltest1;
+drop database if exists x;
 --enable_warnings
 create database mysqltest1;
+set sql_log_bin=0;
+create database x;
+set sql_log_bin=1;
 
 use mysqltest1;
 create table t1 (a int);
 insert into t1 values(9);
+use x;
+create table t1 (a int);
+insert into t1 values(9);
 select * from mysqltest1.t1;
+select * from x.t1;
 sync_slave_with_master;
+#TODO no it is no empty
 show databases like 'mysqltest1'; # should be empty
 select * from test.t1;
+select * from y.t1;
 # cleanup
 connection master;
+use mysqltest1;
 drop table t1;
 drop database mysqltest1;
 sync_slave_with_master;
@@ -224,6 +241,7 @@ SET sql_log_bin= 0;
 DROP DATABASE database_master_temp_01;
 DROP DATABASE database_master_temp_02;
 DROP DATABASE database_master_temp_03;
+DROP DATABASE x;
 SET sql_log_bin= 1;
 
 connection slave;
@@ -231,6 +249,7 @@ SET sql_log_bin= 0;
 DROP DATABASE database_slave_temp_01;
 DROP DATABASE database_slave_temp_02;
 DROP DATABASE database_slave_temp_03;
+DROP DATABASE y;
 SET sql_log_bin= 1;
 
 connection master;

sql/mysqld.cc

@@ -7596,7 +7596,8 @@ mysqld_get_one_option(int optid,
     val= p--;
     while (my_isspace(mysqld_charset, *p) && p > argument)
       *p-- = 0;
-    if (p == argument)
+    /* Db name can be one char also */
+    if (p == argument && my_isspace(mysqld_charset, *p))
     {
       sql_print_error("Bad syntax in replicate-rewrite-db - empty FROM db!\n");
       return 1;