git_config_set_multivar_in_file: handle "unset" errors

git_config_set_multivar_in_file: handle "unset" errors

We pass off to the "_gently" form to do the real work, and
just die() if it returned an error. However, our die message
de-references "value", which may be NULL if the request was
to unset a variable. Nobody using glibc noticed, because it
simply prints "(null)", which is good enough for the test
suite (and presumably very few people run across this in
practice). But other libc implementations (like Solaris) may
segfault.

Let's not only fix that, but let's make the message more
clear about what is going on in the "unset" case.

Reported-by: "Tom G. Christensen" <tgc@jupiterrise.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>