If a run iterator is positioned at a non-terminal statement (UPSERT or
UPDATE), `vy_run_iterator_next()` will iterate over older statements
with the same key using `vy_run_iterator_next_lsn()` to build the key
history. While doing so, it may reach the end of the run file (if the
current key is the last in the run). This would stop iteration
permanently, which is apparently wrong for reverse iterators (LE or LT):
if this happens the run iterator won't return any keys preceding the
last one in the run file. Fix this by removing `vy_run_iterator_stop()`
from `vy_run_iterator_next_lsn()`.

Part of #10109

NO_DOC=bug fix
NO_CHANGELOG=next commit