dev-games/physfs: backport patch to fix PHYSFS_setWriteDir

dev-games/physfs: backport patch to fix PHYSFS_setWriteDir

There was a bug in PHYSFS_setWriteDir() that caused creating an empty
file "$HOME/.lincity-ng" instead of a directory on the first run of
lincity-ng:
https://github.com/lincity-ng/lincity-ng/issues/25

The upstream patch is added to <stable-3.0> branch but no release is
made so far. Backport it.

Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: Stefan Strogin <stefan.strogin@gmail.com>
Closes: https://github.com/gentoo/gentoo/pull/11231
Signed-off-by: James Le Cuirot <chewi@gentoo.org>