Fix buffer_inflate() data loss

Make sure we don't lose data if there's not enough space in the output
buffer for buffer_inflate(). Also switch test-nasdaq-itch41.c to use
mmap() to simplify input buffer management.

Reported-by: Ville Aine <ville.aine@iki.fi>
Signed-off-by: Pekka Enberg <penberg@kernel.org>