t/t5318: introduce failing 'git commit-graph write' tests

When invoking 'git commit-graph' in a corrupt repository, one can cause
a segfault when ancestral commits are corrupt in one way or another.
This is due to two function calls in the 'commit-graph.c' code that may
return NULL, but are not checked for NULL-ness before dereferencing.

Before fixing the bug, introduce two failing tests that demonstrate the
problem. The first test corrupts an ancestral commit's parent to point
to a non-existent object. The second test instead corrupts an ancestral
tree by removing the 'tree' information entirely from the commit. Both
of these cases cause segfaults, each at different lines.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5318-commit-graph.sh

@@ -585,4 +585,47 @@ test_expect_success 'get_commit_tree_in_graph works for non-the_repository' '
 	test_cmp expect actual
 '
 
+test_expect_failure 'corrupt commit-graph write (broken parent)' '
+	rm -rf repo &&
+	git init repo &&
+	(
+		cd repo &&
+		empty="$(git mktree </dev/null)" &&
+		cat >broken <<-EOF &&
+		tree $empty
+		parent 0000000000000000000000000000000000000000
+		author whatever <whatever@example.com> 1234 -0000
+		committer whatever <whatever@example.com> 1234 -0000
+
+		broken commit
+		EOF
+		broken="$(git hash-object -w -t commit --literally broken)" &&
+		git commit-tree -p "$broken" -m "good commit" "$empty" >good &&
+		test_must_fail git commit-graph write --stdin-commits \
+			<good 2>test_err &&
+		test_i18ngrep "unable to parse commit" test_err
+	)
+'
+
+test_expect_failure 'corrupt commit-graph write (missing tree)' '
+	rm -rf repo &&
+	git init repo &&
+	(
+		cd repo &&
+		tree="$(git mktree </dev/null)" &&
+		cat >broken <<-EOF &&
+		parent 0000000000000000000000000000000000000000
+		author whatever <whatever@example.com> 1234 -0000
+		committer whatever <whatever@example.com> 1234 -0000
+
+		broken commit
+		EOF
+		broken="$(git hash-object -w -t commit --literally broken)" &&
+		git commit-tree -p "$broken" -m "good" "$tree" >good &&
+		test_must_fail git commit-graph write --stdin-commits \
+			<good 2>test_err &&
+		test_i18ngrep "unable to get tree for" test_err
+	)
+'
+
 test_done