submodule: fix segmentation fault in submodule--helper clone
The git submodule--helper clone command will fail with a segmentation
fault when given a null url or null path variable. Since these are
required for proper functioning of the submodule--helper clone
subcommand, add checks to prevent running and fail gracefully when
missing.
Update the usage string to reflect the requirement that the --url and
--path "options" are required.
Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>