qcow2: Prevent allocating compressed clusters at offset 0

qcow2: Prevent allocating compressed clusters at offset 0

If the refcount data is corrupted then we can end up trying to
allocate a new compressed cluster at offset 0 in the image, triggering
an assertion in qcow2_alloc_bytes() that would crash QEMU:

  qcow2_alloc_bytes: Assertion `offset' failed.

This patch adds an explicit check for this scenario and a new test
case.

Signed-off-by: Alberto Garcia <[email protected]>
Message-id: fb53467cf48e95ff3330def1cf1003a5b862b7d9.1509718618[email protected]
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Max Reitz <[email protected]>