Avoid unexpected conversion overflow in planner for distant date values.

The "date" type supports a wider range of dates than int64 timestamps do.
However, there is pre-int64-timestamp code in the planner that assumes that
all date values can be converted to timestamp with impunity.  Fortunately,
what we really need out of the conversion is always a double (float8)
value; so even when the date is out of timestamp's range it's possible to
produce a sane answer.  All we need is a code path that doesn't try to
force the result into int64.  Per trouble report from David Rericha.

Back-patch to all supported versions.  Although this is surely a corner
case, there's not much point in advertising a date range wider than
timestamp's if we will choke on such values in unexpected places.

src/backend/utils/adt/selfuncs.c

@@ -3866,8 +3866,7 @@ convert_timevalue_to_scalar(Datum value, Oid typid)
 			return DatumGetTimestamp(DirectFunctionCall1(abstime_timestamp,
 														 value));
 		case DATEOID:
-			return DatumGetTimestamp(DirectFunctionCall1(date_timestamp,
-														 value));
+			return date2timestamp_no_overflow(DatumGetDateADT(value));
 		case INTERVALOID:
 			{
 				Interval   *interval = DatumGetIntervalP(value);