struct pool always needs a non-NULL buf field: it points either to the
actual memory used to store packets, or for vhost-user to the vhost user
memory structure which will contain the packets. We set this pointer
when we initialise the pool. However, clang-tidy (as of 22.1.4, at least)
doesn't realise this in packet_check_range(), causing UB warnings due to
the subtraction of ptr and p->buf.
Clue it in with an assert().
Signed-off-by: David Gibson