qemu: driver: Fix off-by-one in qemuDomainSnapshotDiskDataCollect

Commit f34397e5 introduced a crash-inducing problem when collecting
disk snapshot data, where the array would be filled starting from the
second element.

The code then dereferenced the first one.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -15073,8 +15073,8 @@ qemuDomainSnapshotDiskDataCollect(virQEMUDriverPtr driver,
         if (snapdef->disks[i].snapshot == VIR_DOMAIN_SNAPSHOT_LOCATION_NONE)
             continue;
 
-        ndata++;
         dd = data + ndata;
+        ndata++;
 
         dd->disk = vm->def->disks[i];