more sys_var_str fixes

mysql-test/r/variables.result:
  more tests
mysql-test/t/variables.test:
  more tests
sql/set_var.cc:
  don't crash on NULL

mysql-test/r/variables.result

@@ -384,6 +384,15 @@ select @@session.key_buffer_size;
 ERROR HY000: Variable 'key_buffer_size' is a GLOBAL variable
 set ft_boolean_syntax = @@init_connect;
 ERROR HY000: Variable 'ft_boolean_syntax' is a GLOBAL variable and should be set with SET GLOBAL
+set global ft_boolean_syntax = @@init_connect;
+ERROR 42000: Variable 'ft_boolean_syntax' can't be set to the value of ''
+set init_connect = NULL;
+ERROR HY000: Variable 'init_connect' is a GLOBAL variable and should be set with SET GLOBAL
+set global init_connect = NULL;
+set ft_boolean_syntax = @@init_connect;
+ERROR HY000: Variable 'ft_boolean_syntax' is a GLOBAL variable and should be set with SET GLOBAL
+set global ft_boolean_syntax = @@init_connect;
+ERROR 42000: Variable 'ft_boolean_syntax' can't be set to the value of ''
 select @@global.max_user_connections,@@local.max_join_size;
 @@global.max_user_connections	@@session.max_join_size
 100	200

mysql-test/t/variables.test

@@ -272,6 +272,15 @@ select @@session.key_buffer_size;
 
 --error 1229
 set ft_boolean_syntax = @@init_connect;
+--error 1231
+set global ft_boolean_syntax = @@init_connect;
+--error 1229
+set init_connect = NULL;
+set global init_connect = NULL;
+--error 1229
+set ft_boolean_syntax = @@init_connect;
+--error 1231
+set global ft_boolean_syntax = @@init_connect;
 
 #
 # swap

sql/set_var.cc

@@ -835,15 +835,11 @@ bool sys_var_str::check(THD *thd, set_var *var)
 bool update_sys_var_str(sys_var_str *var_str, rw_lock_t *var_mutex,
 			set_var *var)
 {
-  char *res= 0, *old_value;
-  uint new_length= 0;
-  /* If the string is "", delete old init command */
-  if (var && (new_length= var->value->str_value.length()))
-  {
-    if (!(res= my_strdup_with_length((byte*) var->value->str_value.ptr(),
-				     new_length, MYF(0))))
+  char *res= 0, *old_value=(char *)(var ? var->value->str_value.ptr() : 0);
+  uint new_length= (var ? var->value->str_value.length() : 0);
+  if (!old_value) old_value="";
+  if (!(res= my_strdup_with_length(old_value, new_length, MYF(0))))
     return 1;
-  }
   /*
     Replace the old value in such a way that the any thread using
     the value will work.