qcow2: Fix NULL dereference in qcow2_open() error path (CVE-2014-0146)

qcow2: Fix NULL dereference in qcow2_open() error path (CVE-2014-0146)

The qcow2 code assumes that s->snapshots is non-NULL if s->nb_snapshots
!= 0. By having the initialisation of both fields separated in
qcow2_open(), any error occuring in between would cause the error path
to dereference NULL in qcow2_free_snapshots() if the image had any
snapshots.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 11b128f4062dd7f89b14abc8877ff20d41b28be9)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>