Fix error detection in pfring.NewRing

The cgo documentation states:

> Any C function (even void functions) may be called in a multiple
> assignment context to retrieve both the return value (if any) and the
> C errno variable as an error

See: <https://pkg.go.dev/cmd/cgo>

Furthermore, the manpage for errno states:

> The  value  in  errno is significant only when the return value of the
> call indicated an error (i.e., -1 from most system calls; -1 or NULL
> from most library functions); a function that succeeds is allowed to
> change errno. The value of errno is never set to zero by any system
> call or library function.

See: man 3 errno

Therefore, I believe the value of err MUST be ignored unless we have
something else that reports an error.

Author: Étienne Miret

pfring/pfring.go

@@ -99,7 +99,7 @@ func NewRing(device string, snaplen uint32, flags Flag) (ring *Ring, _ error) {
 	defer C.free(unsafe.Pointer(dev))
 
 	cptr, err := C.pfring_open(dev, C.u_int32_t(snaplen), C.u_int32_t(flags))
-	if cptr == nil || err != nil {
+	if cptr == nil {
 		return nil, fmt.Errorf("pfring NewRing error: %v", err)
 	}
 	ring = &Ring{cptr: cptr}