intel: Avoid the need for most overflow checks by using a scratch page.

The overflow checks were all thoroughly untested, and a bunch of the
ones I'm deleting were pretty broken.  Now, in the case of overflow,
you just decode data of 0xd0d0d0d0, and instr_out prints the warning
message instead.  Note that this still has the same issue of being
under-tested, but at least it's one place instead of per-packet.

A couple of BUFFER_FAIL uses are left where the length to be decoded
could be (significantly) larger than a page, and the decode didn't
just call instr_out (which doesn't dereference data itself unless it's
safe).