makechrootpkg: fix error message

Passing a directory that does not exist to makechrootpkg results in
an error message:

==> ERROR: No chroot dir defined, or invalid path ''

The path is not being printed as the readlink command blanks it if
the directory does not exist. Fix this.

Signed-off-by: Allan McRae <allan@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>