rust: fix compiler warning for confusing lifetimes

rust: fix compiler warning for confusing lifetimes

For example:

error: lifetime flowing from input to output with different syntax can be confusing
   --> htp/src/headers.rs:475:16
    |
475 | fn null(input: &[u8]) -> IResult<&[u8], ParsedBytes> {
    |                ^^^^^             -----  ----------- the lifetimes get resolved as `'_`
    |                |                 |
    |                |                 the lifetimes get resolved as `'_`
    |                this lifetime flows to the output
    |
note: the lint level is defined here
   --> htp/src/lib.rs:3:9

This currently only happens when using the Rust nightly compiler, which
we use for our fuzz builds.