Fix segfault

[serpilliere]

Hi!
This PR fixes #15

The problem is that nfq_get_payload takes a pointer of pointer in parameters in order to return a pointer of the received packet.
The original code uses &*mut libc::c_void to describe this. This is not correct as the value of the pointer will be modified by nfq_get_payload. The correct declaration may be &mut *mut libc::c_void.

This code crashed in release mode due to the optimization made:

• the pointer initialization is null
• a reference on this pointer is given to nfq_get_payload, but as it's given as &, it notes that the pointer value cannot be changed.
• when creating the slice from the pointer value, the optimization creates it using on a null pointer
• (this can be seen if you disassemble the binary)