segfault on invalid 'git grep' invocation

[Matthew Hughes <matthewhughes934@gmail.com>]

Hi all,

It's my first time writing to this mailing list, so apologies in advance
for any mistakes/faux pas.

Summary
=======

Segfault seen when running:

    $ ./git grep --and -e 'pattern' -- not_a_path

Expected Behaviour
==================

The above invocation doesn't appear particularly valid to me, I
accidentally ran it while editing a command. Not sure if this needs
addressing, but perhaps the expected behaviour would be an error message
about it not being valid?

Compare with, running without '--':

    $ ./git grep --and -e 'pattern' not_a_path
    fatal: ambiguous argument 'not_a_path': unknown revision or path not in the working tree.
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'

Running with a valid path:

    $ ./git grep --and -e 'pattern' -- common-main.c
    fatal: Not a valid grep expression

    $ ./git grep --and -e 'pattern' common-main.c
    fatal: Not a valid grep expression


Background
==========

git version:
git version 2.32.0.93.g670b81a890
cpu: x86_64
built from commit: 670b81a890388c60b7032a4f5b879f2ece8c4558
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 4.19.0-16-amd64 #1 SMP Debian 4.19.181-1 (2021-03-19) x86_64
compiler info: gnuc: 8.3
libc info: glibc: 2.28
$SHELL (typically, interactive shell): /bin/bash

Also reproduced on git 2.20.1 (packaged version on Debian 10)

The segfault is raised at free_pattern_expr (x=0x0) at grep.c:825 (on
commit 670b81a890388c60b7032a4f5b879f2ece8c4558)

Regards,
Matt