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

The executor has thrown errors for negative OFFSET values since 8.4 (see
commit bfce56ee), but in a moment of brain
fade I taught the planner that OFFSET with a constant negative value was a
no-op (commit 1a1832eb).  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.

src/backend/optimizer/plan/planner.c

@@ -2335,7 +2335,7 @@ preprocess_limit(PlannerInfo *root, double tuple_fraction,
 			{
 				*offset_est = DatumGetInt64(((Const *) est)->constvalue);
 				if (*offset_est < 0)
-					*offset_est = 0;	/* less than 0 is same as 0 */
+					*offset_est = 0;	/* treat as not present */
 			}
 		}
 		else
@@ -2496,9 +2496,8 @@ limit_needed(Query *parse)
 			{
 				int64		offset = DatumGetInt64(((Const *) node)->constvalue);
 
-				/* Executor would treat less-than-zero same as zero */
-				if (offset > 0)
-					return true;	/* OFFSET with a positive value */
+				if (offset != 0)
+					return true;	/* OFFSET with a nonzero value */
 			}
 		}
 		else