qcow2-refcount: qemu-io crashes on 'discard' command

qemu-io is killed by SIGIOT at the 'discard' command on the image having no refcount information.

Sequence:
1. Unpack test.img and backing_img.qed in the same directory (see the attached archives for images)
2. Make a copy of test.img to copy.img (qemu-io modifies the image before being kill, therefore the image backup is necessary)
3. Run the command

qemu-io copy.img -c 'discard 2210816 2856448'

Result: qemu-io is killed by SIGIOT with the reason:

qemu-io: block/qcow2-refcount.c:468: update_refcount_discard: Assertion `d->bytes + length == new_end - new_start' failed.


The image was generated by the image fuzzer.

qemu.git HEAD: 1d80eb7a680d