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