librbd: fix use-after-free in trash purge on image open error

librbd: fix use-after-free in trash purge on image open error

When ImageState::open() fails with an error other than -ENOENT, it
asynchronously deletes the ImageCtx before returning. The trash purge
loop only skipped the -ENOENT case and fell through on every other
error, dereferencing the already-freed ictx in diff_iterate() and
state->close(). Add the missing continue so a failed open is skipped.

The fall-through dates back to 504f4e78, which split `if (r < 0)
continue` to log non-ENOENT errors but dropped the continue.

Fixes: https://tracker.ceph.com/issues/77836
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>