Do a check of the block total length in pcapng_read_unknown_block().
That means we don't need to do the block length check in
pcapng_read_block(); each block type reader, including the one for
unknown block types, does a check that's as stringent as that block
length check or more stringent, which means any block whose length is
less than the minimum will fail with the same error in both cases.
Fix the message for a too-short NRB.