qcow2: Fix potential qemu-img check crash on 32 bit hosts

This crash was caught with qemu-iotests test case 138.

Commit b6d36de already fixed a few 32 bit truncation bugs that could
cause qemu-img check to allocate too little memory and consequently
it would segfault. On 32 bit hosts, there is one more place that needs
to be fixed because size_t was involved in the calculation and is a
32 bit type there.

Cc: qemu-stable@nongnu.org
Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Michael S. Tsirkin <mst@redhat.com>
