syscall: handle empty address in ReadFrom better

Handle return values from recvfrom correctly when the
kernel decides to not return an address.

Fixes #4636.
Fixes #4352.

R=rsc, mikioh.mikioh, dave
CC=golang-dev
https://golang.org/cl/7058062

src/pkg/syscall/syscall_bsd.go

@@ -450,7 +450,9 @@ func Recvfrom(fd int, p []byte, flags int) (n int, from Sockaddr, err error) {
 	if n, err = recvfrom(fd, p, flags, &rsa, &len); err != nil {
 		return
 	}
-	from, err = anyToSockaddr(&rsa)
+	if rsa.Addr.Family != AF_UNSPEC {
+		from, err = anyToSockaddr(&rsa)
+	}
 	return
 }