download/git: more resilient in case of kill

In case the git backend gets killed right in-between it finished
initialising the repository, but before it could add the remote,
we'd end up with a repository without the 'origin' remote, so we
would not be able to change its URL.

Another case that may happen (like in the build failure, below),
is that the repository was initialised with a previous version
of Buildroot, before the commit e1771926 (download/git: don't
require too-recent git) was applied, and that trepository was
still lying around...

Fixes:
    http://autobuild.buildroot.org/results/25a/25aae054634368fadb265b97ebe4dda809deff6f/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>