Fix segfault when calling git_blame_buffer()

This change makes sure that the hunk is not null before trying to
dereference it. This avoids segfaults, especially when blaming against a
modified buffer (i.e. the index).

Fixes: #5443

src/blame.c

@@ -415,7 +415,7 @@ on_error:
 
 static bool hunk_is_bufferblame(git_blame_hunk *hunk)
 {
-	return git_oid_is_zero(&hunk->final_commit_id);
+	return hunk && git_oid_is_zero(&hunk->final_commit_id);
 }
 
 static int buffer_hunk_cb(

tests/blame/buffer.c

@@ -17,6 +17,32 @@ void test_blame_buffer__cleanup(void)
 	git_repository_free(g_repo);
 }
 
+void test_blame_buffer__index(void)
+{
+	const git_blame_hunk *hunk;
+	const char *buffer = "Hello\nWorld!";
+
+	/*
+	 * We need to open a different file from the ones used in other tests. Close
+	 * the one opened in test_blame_buffer__initialize() to avoid a leak.
+	 */
+	git_blame_free(g_fileblame);
+	g_fileblame = NULL;
+	cl_git_pass(git_blame_file(&g_fileblame, g_repo, "file.txt", NULL));
+
+	cl_git_pass(git_blame_buffer(&g_bufferblame, g_fileblame, buffer, strlen(buffer)));
+	cl_assert_equal_i(2, git_blame_get_hunk_count(g_bufferblame));
+
+	check_blame_hunk_index(g_repo, g_bufferblame, 0,  1, 1, 0, "836bc00b", "file.txt");
+	hunk = git_blame_get_hunk_byline(g_bufferblame, 1);
+	cl_assert(hunk);
+	cl_assert_equal_s("lhchavez", hunk->final_signature->name);
+	check_blame_hunk_index(g_repo, g_bufferblame, 1,  2, 1, 0, "00000000", "file.txt");
+	hunk = git_blame_get_hunk_byline(g_bufferblame, 2);
+	cl_assert(hunk);
+	cl_assert(hunk->final_signature == NULL);
+}
+
 void test_blame_buffer__added_line(void)
 {
 	const git_blame_hunk *hunk;

tests/resources/blametest.git/refs/heads/master

-6653ff42313eb5c82806f145391b18a9699800c7
+836bc00b06cb60eb0f629e237ad2b58adb2cfc7e