Fix heuristic not to be byte-order-dependent.

Fix heuristic not to be byte-order-dependent.

I'm *guessing* that the item being fetched is big-endian; the "vt"
protocol might be old enough that they developed it on (non-Sun386i) Sun
workstations, assuming they didn't just say "this is an Internet
protocol, hence fields are big-endian").

That also prevents problems on platforms that don't support unaligned
accesses.