pretty: handle broken commit headers gracefully

Centralize the parsing of the date and time zone strings in the new
helper function show_ident_date() and make sure it checks the pointers
provided by split_ident_line() for NULL before use.

Reported-by: Ivan Lyapunov <dront78@gmail.com>
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4212-log-corrupt.sh

@@ -0,0 +1,42 @@
+#!/bin/sh
+
+test_description='git log with invalid commit headers'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit foo &&
+
+	git cat-file commit HEAD |
+	sed "/^author /s/>/>-<>/" >broken_email.commit &&
+	git hash-object -w -t commit broken_email.commit >broken_email.hash &&
+	git update-ref refs/heads/broken_email $(cat broken_email.hash)
+'
+
+test_expect_success 'git log with broken author email' '
+	{
+		echo commit $(cat broken_email.hash)
+		echo "Author: A U Thor <author@example.com>"
+		echo "Date:   Thu Jan 1 00:00:00 1970 +0000"
+		echo
+		echo "    foo"
+	} >expect.out &&
+	: >expect.err &&
+
+	git log broken_email >actual.out 2>actual.err &&
+
+	test_cmp expect.out actual.out &&
+	test_cmp expect.err actual.err
+'
+
+test_expect_success 'git log --format with broken author email' '
+	echo "A U Thor+author@example.com+" >expect.out &&
+	: >expect.err &&
+
+	git log --format="%an+%ae+%ad" broken_email >actual.out 2>actual.err &&
+
+	test_cmp expect.out actual.out &&
+	test_cmp expect.err actual.err
+'
+
+test_done