Use literal instead of constant to fix 9.4 build

PG_UINT32_MAX doesn't exist before 9.5. Missed this because I removed
my assert-enabled builds during packaging work.

Fixes #619

src/backend/distributed/executor/multi_router_executor.c

@@ -301,9 +301,8 @@ ExecuteDistributedModify(Task *task)
 		Assert(currentAffectedTupleCount >= 0);
 
 #if (PG_VERSION_NUM < 90600)
-
 		/* before 9.6, PostgreSQL used a uint32 for this field, so check */
-		Assert(currentAffectedTupleCount <= PG_UINT32_MAX);
+		Assert(currentAffectedTupleCount <= 0xFFFFFFFF);
 #endif
 
 		if ((affectedTupleCount == -1) ||