lib-mail: message_address_write() - don't crash with NULL address

message_address_parse() can return NULL on empty address, so writing it
should produce empty address as well. Broken by
1558129

src/lib-mail/message-address.c

@@ -430,6 +430,16 @@ void message_address_write(string_t *str, const struct message_address *addr)
 	const char *tmp;
 	bool first = TRUE, in_group = FALSE;
 
+	if (addr == NULL)
+		return;
+
+	/* <> path */
+	if (addr->mailbox == NULL && addr->domain == NULL) {
+		i_assert(addr->next == NULL);
+		str_append(str, "<>");
+		return;
+	}
+
 	/* a) mailbox@domain
 	   b) name <@route:mailbox@domain>
 	   c) group: .. ; */

src/lib-mail/test-message-address.c

@@ -311,6 +311,13 @@ static void test_message_address(void)
 		    cmp_addr(addr, &group_suffix));
 	test_assert(strcmp(str_c(str), "group:;") == 0);
 	test_end();
+
+	test_begin("message address parsing empty string");
+	test_assert(message_address_parse(unsafe_data_stack_pool, &uchar_nul, 0, 10, TRUE) == NULL);
+	str_truncate(str, 0);
+	message_address_write(str, NULL);
+	test_assert(str_len(str) == 0);
+	test_end();
 }
 
 int main(void)