fsck: avoid looking at NULL blob->object

Commit 159e7b080b (fsck: detect gitmodules files,
2018-05-02) taught fsck to look at the content of
.gitmodules files. If the object turns out not to be a blob
at all, we just complain and punt on checking the content.
And since this was such an obvious and trivial code path, I
didn't even bother to add a test.

Except it _does_ do one non-trivial thing, which is call the
report() function, which wants us to pass a pointer to a
"struct object". Which we don't have (we have only a "struct
object_id"). So we erroneously pass a NULL object to
report(), which gets dereferenced and causes a segfault.

It seems like we could refactor report() to just take the
object_id itself. But we pass the object pointer along to
a callback function, and indeed this ends up in
builtin/fsck.c's objreport() which does want to look at
other parts of the object (like the type).

So instead, let's just use lookup_unknown_object() to get
the real "struct object", and pass that.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -1032,7 +1032,8 @@ int fsck_finish(struct fsck_options *options)
 
 		blob = lookup_blob(oid);
 		if (!blob) {
-			ret |= report(options, &blob->object,
+			struct object *obj = lookup_unknown_object(oid->hash);
+			ret |= report(options, obj,
 				      FSCK_MSG_GITMODULES_BLOB,
 				      "non-blob found at .gitmodules");
 			continue;

t/t7415-submodule-names.sh

@@ -148,4 +148,22 @@ test_expect_success 'fsck detects symlinked .gitmodules file' '
 	)
 '
 
+test_expect_success 'fsck detects non-blob .gitmodules' '
+	git init non-blob &&
+	(
+		cd non-blob &&
+
+		# As above, make the funny tree directly to avoid index
+		# restrictions.
+		mkdir subdir &&
+		cp ../.gitmodules subdir/file &&
+		git add subdir/file &&
+		git commit -m ok &&
+		git ls-tree HEAD | sed s/subdir/.gitmodules/ | git mktree &&
+
+		test_must_fail git fsck 2>output &&
+		grep gitmodulesBlob output
+	)
+'
+
 test_done