Bug fix:

  SET character_set_connection=newcharset;
  SELECT hex('text');

The above sequence failed in some cases.

mysql-test/r/ctype_recoding.result

@@ -70,3 +70,11 @@ SHOW TABLES IN
 Tables_in_
 SET CHARACTER SET koi8r;
 DROP DATABASE ;
+SET NAMES koi8r;
+SELECT hex('');
+hex('')
+D4C5D3D4
+SET character_set_connection=cp1251;
+SELECT hex('');
+hex('')
+F2E5F1F2

mysql-test/t/ctype_recoding.test

@@ -45,3 +45,8 @@ SHOW TABLES;
 SHOW TABLES IN ;
 SET CHARACTER SET koi8r;
 DROP DATABASE ;
+
+SET NAMES koi8r;
+SELECT hex('');
+SET character_set_connection=cp1251;
+SELECT hex('');

sql/set_var.cc

@@ -1343,6 +1343,7 @@ bool sys_var_character_set::check(THD *thd, set_var *var)
 bool sys_var_character_set::update(THD *thd, set_var *var)
 {
   ci_ptr(thd,var->type)[0]= var->save_result.charset;
+  thd->update_charset();
   return 0;
 }