Avoid possible crash while finishing up a heap rewrite.

Avoid possible crash while finishing up a heap rewrite.

end_heap_rewrite was not careful to ensure that the target relation
is open at the smgr level before performing its final smgrimmedsync.
In ordinary cases this is no problem, because it would have been
opened earlier during the rewrite.  However a crash can be reproduced
by re-clustering an empty table with CLOBBER_CACHE_ALWAYS enabled.

Although that exact scenario does not crash in v13, I think that's
a chance result of unrelated planner changes, and the problem is
likely still reachable with other test cases.  The true proximate
cause of this failure is commit c6b92041d, which replaced a call to
heap_sync (which was careful about opening smgr) with a direct call
to smgrimmedsync.  Hence, back-patch to v13.

Amul Sul, per report from Neha Sharma; cosmetic changes
and test case by me.

Discussion: https://postgr.es/m/CANiYTQsU7yMFpQYnv=BrcRVqK_3U3mtAzAsJCaqtzsDHfsUbdQ@mail.gmail.com