vinyl: fix double free in cache iterator restore

We drop a reference to itr->curr_stmt before restoring the iterator
position so we must not drop it while iterating over the cache tree.
This mistake results in funky crashes in various places, e.g. in
vy_stmt_compare(). Shame on me: I missed that during self review.

Fixes: 073c8d07 ("vinyl: rework cache iterator restore")