pinentry-tty: fix segfault on MESSAGE or CONFIRM

* tty/pinentry-tty.c: avoid prompting for a PIN when one was not asked
  for.

Before this, pinentry-tty would segfault when asked for MESSAGE or
CONFIRM:

0 dkg@alice:~$ pinentry-tty
OK Your orders please
SETDESC testing testing
OK
MESSAGE
testing testing
PIN? :
Segmentation fault
139 dkg@alice:~$