lib-smtp: smtp-client-command - Generate multiple callbacks when required in smtp_cli...

lib-smtp: smtp-client-command - Generate multiple callbacks when required in smtp_client_command_fail*()

This requirement applies to the LTMP DATA command when more than a single
recipient is approved.

This caused a crash in the LMTP service occurring when the proxy client
connection was lost during the DATA command while more than a single RCPT
command was expected earlier. This situation caused an early free/dangling
pointer for the DATA command struct, subsequently causing a crash at
deinitialization of the proxy.