fix panic on 256 bits RC4 keys

Found this bug while fuzzing the crate.

The spec says that in the ecryption dictonary, /Length is the length of the
key, and should be between 40 to 128 bits. However, if we specify 256 the
parser still accepts it but panics in Decoder::decrypt() while performing
the b) step.

The b) step is implemented by truncating the key length to a maximum of 128
bits (16 bytes) before concatenating the object number and generation of the
string being decrypted.

However, while computing the size of n, Decoder::decode() does not truncate the
key size to 16, and this leads to an out of bound panic.

Note that I can't find the reason for this trucation to 16 bytes in the b) step.
The spec does require truncating the key to 16 bytes, but that's in the d)
step which is where we truncate the resulting hash.

Maybe a better fix would be to reject a /Length of 256 bits in the parser instead.

pdf/src/crypt.rs

@@ -562,7 +562,7 @@ impl Decoder {
             CryptMethod::V2 => {
                 // b)
                 let mut key = [0; 16 + 5];
-                let n = self.key_size;
+                let n = self.key().len();
                 key[..n].copy_from_slice(self.key());
                 key[n..n + 3].copy_from_slice(&id.id.to_le_bytes()[..3]);
                 key[n + 3..n + 5].copy_from_slice(&id.gen.to_le_bytes()[..2]);