[pnet] Compiler optimisation leads to SEGFAULT

Affected versions of the pnet crate were optimized out by compiler, which caused dereference of uninitialized file descriptor which caused segfault.

References

• https://github.com/libpnet/libpnet/issues/449
• https://github.com/libpnet/libpnet/pull/455
• https://rustsec.org/advisories/RUSTSEC-2019-0037.html
• https://github.com/advisories/GHSA-r6ff-2q3c-v3pv