[RFC PATCH 09/15] add-patch: assert parse_diff() expectations with BUG()

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Assert that this code added in [1], [2] and other related commits
expects that once we see a "diff " line we should have a non-NULL
"file_diff" and "hunk".

In practice this would have always been the case, as we are parsing
our own "diff" output, but e.g. GCC v12's -fanalyzer doesn't know
that, and will alert us that in the "else if" and below in this
function we could be dereferencing NULL if we were processing anything
except our expected input.

1. f6aa7ecc343 (built-in add -i: start implementing the `patch`
   functionality in C, 2019-12-13)
2. 80399aec5ab (built-in add -p: support multi-file diffs, 2019-12-13)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 add-patch.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/add-patch.c b/add-patch.c
index 55d719f7845..087bf317b07 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -478,11 +478,16 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 	while (p != pend) {
 		char *eol = memchr(p, '\n', pend - p);
 		const char *deleted = NULL, *mode_change = NULL;
+		const char *const diff_l = "diff ";
+		int is_diff_line = starts_with(p, diff_l);
 
 		if (!eol)
 			eol = pend;
 
-		if (starts_with(p, "diff ")) {
+		if (!is_diff_line && (!file_diff || !hunk))
+			BUG("expected '%s' line to follow a '%s' line", p, diff_l);
+
+		if (is_diff_line) {
 			complete_file(marker, hunk);
 			ALLOC_GROW_BY(s->file_diff, s->file_diff_nr, 1,
 				   file_diff_alloc);
-- 
2.36.1.1124.g577fa9c2ebd