v1 parsing can crash on invalid inputs

[howardjohn]

Describe the bug
During some testing I found
ppp::HeaderResult::parse(&[116, 3, 18, 13]) will result in a panic due to an out-of-bounds array lookup here

ppp/src/v1/mod.rs

Line 168 in 28c5db9

let header = from_utf8(&input[..length])?;

Expected behavior
Should return an error, not panic