wireguard-go bug with IPv6-less kernels

[Kent Friis]

I've been setting up wireguard-go on an old Linux kernel (no module),
and hit a bug with ipv6-less kernels. The create6() function returns 0
as the port number when an error occurs, so even though CreateBind
checks for EAFNOSUPPORT, it still ends up with port getting set to zero,
resulting in a random port.

I solved it by returning port instead of zero - as far as I can see
this shouldn't break anything, as CreateBind will return 0 anyway on
errors other than EAFNOSUPPORT.

I did not check whether the same bug exists in conn_default.go

- Kent

--- conn_linux.go.old	2018-09-05 23:11:19.407372785 +0200
+++ conn_linux.go	2018-09-07 16:58:28.971914271 +0200
@@ -335,7 +335,7 @@
 	)
 
 	if err != nil {
-		return -1, 0, err
+		return -1, port, err
 	}
 
 	addr := unix.SockaddrInet4{
@@ -366,7 +366,7 @@
 		return unix.Bind(fd, &addr)
 	}(); err != nil {
 		unix.Close(fd)
-		return -1, 0, err
+		return -1, port, err
 	}
 
 	return fd, uint16(addr.Port), err
@@ -383,7 +383,7 @@
 	)
 
 	if err != nil {
-		return -1, 0, err
+		return -1, port, err
 	}
 
 	// set sockopts and bind
@@ -425,7 +425,7 @@
 
 	}(); err != nil {
 		unix.Close(fd)
-		return -1, 0, err
+		return -1, port, err
 	}
 
 	return fd, uint16(addr.Port), err