unix: stop counting trailing NUL for abstract addresses starting with NUL

Changes trailing-NUL-counting behavior for abstract addresses
starting with the NUL character to be the same as abstract
addresses starting with the @ character.

For golang/go#63579.

Change-Id: I2f26de4bcf614c4635ad188b1afa3d14ebd9a95f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/535955
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

unix/syscall_internal_linux_test.go

@@ -517,7 +517,7 @@ func TestSockaddrUnix_sockaddr(t *testing.T) {
 			slen: 2, // family (uint16)
 		},
 		{
-			name: "abstract",
+			name: "abstract_starting_with_at",
 			sa: &SockaddrUnix{
 				Name: "@",
 			},
@@ -526,6 +526,16 @@ func TestSockaddrUnix_sockaddr(t *testing.T) {
 			},
 			slen: 3, // family (uint16) + NULL
 		},
+		{
+			name: "abstract_starting_with_null",
+			sa: &SockaddrUnix{
+				Name: "\x00",
+			},
+			raw: &RawSockaddrUnix{
+				Family: AF_UNIX,
+			},
+			slen: 3, // family (uint16) + NULL
+		},
 		{
 			name: "named",
 			sa: &SockaddrUnix{