vinyl: fix crash if space is dropped while space.get is reading from it

In contrast to vinyl_iterator, vinyl_index_get doesn't take a reference
to the LSM tree while reading from it. As a result, if the LSM tree is
dropped in the meantime, vinyl_index_get will crash. Fix this issue by
surrounding vy_get with vy_lsm_ref/unref.

Closes #4109