Avoid an infinite loop by ensuring that the amount of bytes read is greater
than 0 in MDXFileChunk when calculating the checksum

This edgecase can be triggered if the file is truncated while the checksum
is being calculated (i.e. the EOF is reached)

Differential Revision: https://reviews.freebsd.org/D2351 (patch by darius)
PR: 196694
Reviewed by: delphij, ngie
Submitted by: Daniel O'Connor <darius@dons.net.au>
Sponsored by: EMC / Isilon Storage Division