MDEV-36765, followup 1: acl: handle json_unescape errors without crashing

json_unescape can return negative numbers, and with
so we should free the buffer allocated.

Also handle the NULL value in unsafe_str by not de-referencing
NULL.

Author: grooverdan

sql/sql_acl.cc

@@ -1665,7 +1665,7 @@ class User_table_json: public User_table
     set_int_value("version_id", (longlong) MYSQL_VERSION_ID);
   }
   const char *unsafe_str(const char *s) const
-  { return s[0] ? s : NULL; }
+  { return s ? (s[0] ? s : NULL) : NULL; }
 
   SSL_type get_ssl_type () const override
   { return (SSL_type)get_int_value("ssl_type"); }
@@ -1764,6 +1764,8 @@ class User_table_json: public User_table
     if (get_value(key, JSV_STRING, &value_start, &value_len))
       return "";
     char *ptr= (char*)alloca(value_len);
+    if (!ptr)
+      return NULL;
     int len= json_unescape(m_table->field[2]->charset(),
                            (const uchar*)value_start,
                            (const uchar*)value_start + value_len,