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

lib/libmd/mdXhl.c

@@ -74,7 +74,7 @@ MDXFileChunk(const char *filename, char *buf, off_t ofs, off_t len)
 			i = read(f, buffer, sizeof(buffer));
 		else
 			i = read(f, buffer, n);
-		if (i < 0) 
+		if (i <= 0) 
 			break;
 		MDXUpdate(&ctx, buffer, i);
 		n -= i;