MDEV-4801 - Server crashes in my_strdup on setting

            innodb_ft_user_stopword_table to DEFAULT

Setting plugin string variable with PLUGIN_VAR_MEMALLOC flag
to NULL causes server crash.

mysql-test/suite/sys_vars/r/innodb_ft_user_stopword_table_basic.result:
  Reset innodb_ft_user_stopword_table. Also tests MDEV-4801.
mysql-test/suite/sys_vars/t/innodb_ft_user_stopword_table_basic.test:
  Reset innodb_ft_user_stopword_table. Also tests MDEV-4801.
sql/sql_plugin.cc:
  When we got NULL value, do not strdup(NULL).

mysql-test/suite/sys_vars/r/innodb_ft_user_stopword_table_basic.result

@@ -27,3 +27,4 @@ set global innodb_ft_user_stopword_table=1e1;
 ERROR 42000: Incorrect argument type to variable 'innodb_ft_user_stopword_table'
 set global innodb_ft_user_stopword_table='Salmon';
 ERROR 42000: Variable 'innodb_ft_user_stopword_table' can't be set to the value of 'Salmon'
+SET @@session.innodb_ft_user_stopword_table=@start_global_value;

mysql-test/suite/sys_vars/t/innodb_ft_user_stopword_table_basic.test

@@ -35,3 +35,4 @@ set global innodb_ft_user_stopword_table=1e1;
 --error ER_WRONG_VALUE_FOR_VAR
 set global innodb_ft_user_stopword_table='Salmon';
 
+SET @@session.innodb_ft_user_stopword_table=@start_global_value;

sql/sql_plugin.cc

@@ -2700,13 +2700,16 @@ static void update_func_longlong(THD *thd, struct st_mysql_sys_var *var,
 static void update_func_str(THD *thd, struct st_mysql_sys_var *var,
                              void *tgt, const void *save)
 {
-  char *old= *(char **) tgt;
-  *(char **)tgt= *(char **) save;
+  char *value= *(char**) save;
   if (var->flags & PLUGIN_VAR_MEMALLOC)
   {
-    *(char **)tgt= my_strdup(*(char **) save, MYF(0));
+    char *old= *(char**) tgt;
+    if (value)
+      *(char**) tgt= my_strdup(value, MYF(0));
     my_free(old);
   }
+  else
+    *(char**) tgt= value;
 }