gnu: opensmtpd: Fix crash on table authentication.

* gnu/packages/patches/opensmtpd-fix-crash.patch: New patch.
* gnu/packages/mail.scm (opensmtpd)[source]: Use it.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Tobias Geerinckx-Rice <me@tobias.gr>