os: Make sure big requests have sufficient length.

A client can send a big request where the 32B "length" field has value
0. When the big request header is removed and the length corrected,
the value will underflow to 0xFFFFFFFF.  Functions processing the
request later will think that the client sent much more data and may
touch memory beyond the receive buffer.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 9c236850)