qemu-img: Assert for 'amend' command and the fuzzed image

'qemu-img amend' failed with the assert on the fuzzed image.

Sequence:
 1. Unpack the attached archive, make a copy of test.img
 2. Put copy.img and backing_img.vdi in the same directory
 3. Execute
   qemu-img amend -o compat=0.10 -f qcow2 copy.img

Result: qemu-img was killed by SIGIOT with the reason:

qemu-img: block/qcow2-cluster.c:1598: expand_zero_clusters_in_l1: Assertion `(cluster_index >= 0) && (cluster_index < *nb_clusters)' failed.

qemu.git HEAD 2d591ce2aeebf