This commit changes the `Hdlc::Decoder` implementation to report
non-zero length received frames shorter than FCS size as an error
`OT_ERROR_PARSE` (instead of ignoring such frames during decoding).
Since the recent changes in `Hdlc::Decoder` buffer model delegates
the management of the frame buffer to the user of `Decoder`,
clearing frame buffer after an error should be be performed by the
`Decoder` user from the `FrameHandler` callback. The change in this
commit addresses an issue where a small (single-byte) frame would
not have been cleared from the decoder buffer and included in the
next frame. This commit also updates the `test_hdlc` unit test to
cover small frame error.
Abtin Keshavarzian
7dc645a856