sql_class: incorrect assignment in Security_context::destroy

Found by Coverity (id 971843).
Signed-off-by: Daniel Black <daniel.black@au.ibm.com>

sql/sql_class.cc

@@ -3670,7 +3670,7 @@ void Security_context::destroy()
   if (external_user)
   {
     my_free(external_user);
-    user= NULL;
+    external_user= NULL;
   }
 
   my_free(ip);