smtp: check if we have a current transaction

smtp: check if we have a current transaction

Ticket: 4948

This is not the perfect solution, but it prevents to trigger
the assert, and keep the assert.
A better solution would need to create transaction from
the reponse parsing, in case a later command was buffered and
not answered. But this would not be enough as NoNewTx prevents
the creation of a new transaction for RSET...

(cherry picked from commit 4247605d874113ef13003816007e0575219d065e)