PM: hibernate: Do not initialize error in snapshot_write_next()

The error variable in snapshot_write_next() gets a value before it is
used, so don't initialize it to 0 upfront.

Signed-off-by: Li zeming <zeming@nfschina.com>
[ rjw: Subject and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>