add-patch: plug a leak handling the '/' command

add-patch: plug a leak handling the '/' command

Plug a leak we have since d6cf873340 (built-in add -p: implement the '/'
("search regex") command, 2019-12-13).

This leak can be triggered with:

    $ printf "A\n\nB\n" >file
    $ git add file && git commit -m file
    $ printf "AA\n\nBB\n" >file
    $ printf "s\n/ .\n" >lines
    $ git add -p <lines

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>