Merge branch 'tb/submodule-null-deref-fix'

"git submodule" code trusted the data coming from the config (and
the in-tree .gitmodules file) too much without validating, leading
to NULL dereference if the user mucks with a repository (e.g.
submodule.<name>.url is removed).  This has been corrected.

* tb/submodule-null-deref-fix:
  builtin/submodule--helper.c: handle missing submodule URLs

diff --cc builtin/submodule--helper.c
Simple merge