ewah: adjust callers of ewah_read_mmap()

ewah: adjust callers of ewah_read_mmap()

The return value of ewah_read_mmap() is now an ssize_t,
since we could (in theory) process up to 32GB of data. This
would never happen in practice, but a corrupt or malicious
.bitmap or index file could convince us to do so.

Let's make sure that we don't stuff the value into an int,
which would cause us to incorrectly move our pointer
forward.  We'd always move too little, since negative values
are used for reporting errors. So the worst case is just
that we end up reporting a corrupt file, not an
out-of-bounds read.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>