session: su left dangling credentials object on stack

Box.session.su() worked like following: check user
existence, create its credentials on the stack, check
the function, call the function, destroy the
credentials, restore the old credentials.

After creating the credentials on the stack the
function check could raise a Lua error. It led to the
credentials object not being destroyed. As a result,
user.credentials_list was pointing at invalid memory.

Now there is no errors between creating the temporary
credentials and its destruction.

Closes #4597

(cherry picked from commit 2bb8d1ea)