fix: avoid panic on invalid signature input

[nacardin]

Fixes two denial-of-service bugs in the signature verification path.

The library parses the Signature and Signature-Input headers of incoming
requests before any cryptographic check runs, so the bytes it sees are fully
attacker-controlled. Two of those parse steps could panic on malformed input:

1. A negative "created" or "expires" timestamp. These are valid Structured
   Field integers but were converted to u64 with .unwrap(), so a value like
   created=-1 panicked.

2. An undefined @-prefixed derived component name. The
   name lookup called panic! on anything it didn't recognize.

Either one lets an unauthenticated client crash the verifier just by sending a
crafted header.

The fix makes both paths return a normal parse error instead of panicking.