add null terminator to c-string passed to 'libc::open'

[JOE1994]

Hello 🦀 , this PR appends a null-terminator(\0) to c-string fed to libc::open.

Function signature of libc::open is as follows.
open(path: *const c_char, oflag: c_int, ...) -> c_int

Buffer-over-read could happen when the libc implementation relies on path terminating with a null byte.

In src/phy/sys/tap_interface.rs, a string fed to libc::open is appended with a null terminator.
This PR is to do the same thing in src/phy/sys/bpf.rs.

smoltcp/src/phy/sys/tap_interface.rs

Line 21 in bdfa442

let lower = libc::open("/dev/net/tun\0".as_ptr() as *const libc::c_char,

Thank you for reviewing this PR 😺

[Dirbaio]

This is indeed a nasty bug, thanks for the PR!