libblkid: Avoid OOB access on illegal ZFS superblocks

libblkid: Avoid OOB access on illegal ZFS superblocks

64 bit systems can trigger an out of boundary access while performing
a ZFS superblock probe.

This happens due to a possible integer overflow while calculating
the remaining available bytes. The variable is of type "int" and the
string length is allowed to be larger than INT_MAX, which means that
avail calculation can overflow, circumventing the "avail < 0" check and
therefore accessing memory outside the "buff" array later on.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>