util/signer/gnubby.c: set home dir correctly
util/signer/gnubby.c: set home dir correctly

If HOME is not set in the environment, the variable 'home' was getting
looked up, but not set.  This sets the variable.

From https://scan.coverity.com/projects/chromium-ec :
    CID61407: Dereference after null check

BUG=chromium:632768
TEST=Built all boards
BRANCH=None

Previously fixed in commit 4f6f505900a

Change-Id: I77614ed96b5247fc7c6b08d810ea87150ff3adfd
Signed-off-by: Martin Roth <79230511ccf79da3f7e9e82dbcc9c1ec2f734fa0@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/390411
Commit-Ready: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@chromium.org>
Tested-by: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@chromium.org>
Reviewed-by: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@chromium.org>
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec