qemu-img: 'amend -o compat=0.10' command failed with segfault on the fuzzed image

qemu-img amend -o compat=0.10' failed with a segmentation fault on the fuzzed image.

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

Result: qemu-img was killed by SIGSEGV.

Traces can be found in the attached archive.

qemu.git HEAD 2d591ce2aeebf