Fix assert when there is no workdir in decverify

Authored by aheinecke on Jan 30 2018, 8:08 AM.

Description

Fix assert when there is no workdir in decverify

We do not have a workdir or anything to move when verifying
a detached signature.
This error was introduced in 9d1ebcb1e and not yet released,
except in Gpg4win.

Details