submodule--helper update-clone: abort gracefully on missing .gitmodules

submodule--helper update-clone: abort gracefully on missing .gitmodules

When there is no .gitmodules file availabe to initialize a submodule
from, `submodule_from_path` just returns NULL. We need to check for
that and abort gracefully.

When `git submodule update` was implemented in shell, this error out
with the warning

    Submodule path '%s' not initialized
    Maybe you want to use 'update --init'?

Replicate that behavior for now instead of crashing.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>