MINOR: log/cbor: _lf_cbor_encode_byte() explicitly requires non-NULL ctx (again)

MINOR: log/cbor: _lf_cbor_encode_byte() explicitly requires non-NULL ctx (again)

The BUG_ON() statement that was added in 9bdea51 ("MINOR: log/cbor:
_lf_cbor_encode_byte() explicitly requires non-NULL ctx") isn't
sufficient as Coverity still thinks the lf_buildctx itself may be NULL
as shown in GH #2554. In fact the original reports complains about the
lf_buildctx itself and I didn't understand it properly, let's add another
check in the BUG_ON() to ensure both cbor_ctx and cbor_ctx->ctx are not
NULL since it is not expected if used properly.