Because IEEE802.15.4 uses a lot of compression in its frame, fuzzing it
is maybe a good idea. Adding this fuzz target showed that some frame
methods were panicking. `check_len` now checks if accessors will panic
or not.
I ran the fuzzer for about 15 minutes and nothing showed up.
We assume the FCS is checked by lower layers or by hardware.
- Makes it consistent with Ethernet mediums, where we don't handle the FCS either.
- Linux ieee802154 raw sockets send/receive packets without the FCS.