grep.c: refactor free_grep_patterns()

grep.c: refactor free_grep_patterns()

Refactor the free_grep_patterns() function to split out the freeing of
the "struct grep_pat" it contains. Right now we're only freeing the
"pattern_list", but we should be freeing another member of the same
type, which we'll do in the subsequent commit.

Let's also replace the "return" if we don't have an
"opt->pattern_expression" with a conditional call of
free_pattern_expr().

Before db84376f981 (grep.c: remove "extended" in favor of
"pattern_expression", fix segfault, 2022-10-11) the pattern here was:

	if (!x)
		return;
	free_pattern_expr(y);

While at it, instead of:

	if (!x)
		return;
	free_pattern_expr(x);

Let's instead do:

	if (x)
		free_pattern_expr(x);

This will make it easier to free additional members from
free_grep_patterns() in the future.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>