bitbake -g -u depexp <package> segfaults when DISPLAY is not set
properly. Fix it with a proper check.
[YOCTO #7299]
(Bitbake rev: f35e9bd7b59c180fe9a3d9177efb57b92d9cd373)
Signed-off-by: Maxin B. John <maxin.john@enea.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>