Re-enable error for "SELECT ... OFFSET -1".

The executor has thrown errors for negative OFFSET values since 8.4 (see
commit bfce56eea45b1369b7bb2150a150d1ac109f5073), but in a moment of brain
fade I taught the planner that OFFSET with a constant negative value was a
no-op (commit 1a1832eb085e5bca198735e5d0e766a3cb61b8fc).  Reinstate the
former behavior by only discarding OFFSET with a value of exactly 0.  In
passing, adjust a planner comment that referenced the ancient behavior.

Back-patch to 9.3 where the mistake was introduced.