add: plug a leak on interactive_add

add: plug a leak on interactive_add

Plug a leak we have since 5a76aff1a6 (add: convert to use
parse_pathspec, 2013-07-14).

This leak can be triggered with:
    $ git add -p anything

Fixing this leak allows us to mark as leak-free the following tests:

    + t3701-add-interactive.sh
    + t7514-commit-patch.sh

Mark them with "TEST_PASSES_SANITIZE_LEAK=true" to notice and fix
promply any new leak that may be introduced and triggered by them in the
future.

Signed-off-by: Rubén Justo <redacted>
Signed-off-by: Junio C Hamano <redacted>