Fix compiler warning in rangetypes_spgist.c.

On gcc 7.2.0, comparing pointer to (Datum) 0 produces a warning.
Treat it as a simple pointer to avoid that; this is more consistent
with comparable code elsewhere, anyway.

Tomas Vondra

Discussion: https://postgr.es/m/99410021-61ef-9a9a-9bc8-f733ece637ee@2ndquadrant.com

src/backend/utils/adt/rangetypes_spgist.c

@@ -556,7 +556,7 @@ spg_range_quad_inner_consistent(PG_FUNCTION_ARGS)
 					 * for lower or upper bounds to be adjacent. Deserialize
 					 * previous centroid range if present for checking this.
 					 */
-					if (in->traversalValue != (Datum) 0)
+					if (in->traversalValue)
 					{
 						prevCentroid = DatumGetRangeTypeP(in->traversalValue);
 						range_deserialize(typcache, prevCentroid,