mailmap: fix check-mailmap with full mailmap line

I recently had reported to me a crash from a coworker using the recently
added sendemail mailmap support:

  3724814 Segmentation fault      (core dumped) git check-mailmap "[email protected]"

This appears to happen because of the NULL pointer name passed into
map_user(). Fix this by passing "" instead of NULL so that we have a
valid pointer.

Signed-off-by: Jacob Keller <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jacob-keller

builtin/check-mailmap.c

@@ -35,7 +35,7 @@ static void check_mailmap(struct string_list *mailmap, const char *contact)
 		mail = ident.mail_begin;
 		maillen = ident.mail_end - ident.mail_begin;
 	} else {
-		name = NULL;
+		name = "";
 		namelen = 0;
 		mail = contact;
 		maillen = strlen(contact);

t/t4203-mailmap.sh

@@ -114,6 +114,18 @@ test_expect_success 'check-mailmap --stdin simple address: no mapping' '
 	test_cmp expect actual
 '
 
+test_expect_success 'check-mailmap name and address: mapping' '
+	test_when_finished "rm .mailmap" &&
+	cat >.mailmap <<-EOF &&
+	Bug Reports <[email protected]> Bugs <[email protected]>
+	EOF
+	cat >expect <<-EOF &&
+	<[email protected]>
+	EOF
+	git check-mailmap "[email protected]" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'No mailmap' '
 	cat >expect <<-EOF &&
 	$GIT_AUTHOR_NAME (1):