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>

add-patch.c

@@ -1646,6 +1646,7 @@ soft_increment:
 				err(s, _("No hunk matches the given pattern"));
 				break;
 			}
+			regfree(&regex);
 			hunk_index = i;
 		} else if (s->answer.buf[0] == 's') {
 			size_t splittable_into = hunk->splittable_into;