submodule add: sanity check existing .gitmodules

"git submodule add" tries to find if a submodule with the same name
already exists at a different path, by looking up an entry in the
.gitmodules file.  If the entry in the file is incomplete, e.g.,
when the submodule.<name>.something variable is defined but there is
no definition of submodule.<name>.path variable, it accesses the
missing .path member of the submodule structure and triggers a
segfault.

A brief audit was done to make sure that the code does not assume
members other than those that are absolutely certain to exist: a
submodule obtained by submodule_from_name() should have .name
member, while a submodule obtained by submodule_from_path() should
also have .path as well as .name member, and we cannot assume
anything else.  Luckily, the module_add() codepath was the only
problematic one.  It is fairly recent code that comes from 1fa06ced
(submodule: prevent overwriting .gitmodules on path reuse,
2025-07-24).

A helper used by update_submodule() seems to assume that its call to
submodule_from_path() always yields a submodule object without a
failure, which seems to rely on the caller making sure it is the
case.  Leave an assert() with a NEEDSWORK comment there for future
developers to make sure the assumption actually holds.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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