Fix error detection in pfring.NewRing

[EtienneMiret]

Fixes #147

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.

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[karlyeurl]

Hello,

If the error is to be ignored, isn't it better to avoid assigning it in the first place?

Something like cptr, _ := C.pfring_open(dev, C.u_int32_t(snaplen), C.u_int32_t(flags)) would make it clear that we do not care about the error returned by pfring_open().

[EtienneMiret]

Hi @karlyeurl!

Actually, we do care about it when cptr is nil. This means the pfring_open() failed, and in that case we are allowed (and should) look at the C errno variable to have more details about which error occurred.