Fix LIMIT/OFFSET for null limit values. This worked before 8.2 but was broken

Fix LIMIT/OFFSET for null limit values.  This worked before 8.2 but was broken
by the change to make limit values int8 instead of int4.  (Specifically, you
can do DatumGetInt32 safely on a null value, but not DatumGetInt64.)  Per
bug #2803 from Greg Johnson.