submodule: handle NULL value when parsing submodule.*.branch

submodule: handle NULL value when parsing submodule.*.branch

We record the submodule branch config value as a string, so config that
uses an implicit bool like:

  [submodule "foo"]
  branch

will cause us to segfault. Note that unlike most other config-parsing
bugs of this class, this can be triggered by parsing a bogus .gitmodules
file (which we might do after cloning a malicious repository).

I don't think the security implications are important, though. It's
always a strict NULL dereference, not an out-of-bounds read or write. So
we should reliably kill the process. That may be annoying, but the
impact is limited to the attacker preventing the victim from
successfully using "git clone --recurse-submodules", etc, on the
malicious repo.

The "branch" entry is the only one with this problem; other strings like
"path" and "url" already check for NULL.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>