Fix coverity complaint about commit 40d964ec.

The coverity complained that dividing integer expressions and then
converting the integer quotient to type "double" would lose fractional
part.  Typecasting one of the arguments of expression with double should
fix the report.

Author: Mahendra Singh Thalor
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/20200329224818.6phnhv7o2q2rfovf@alap3.anarazel.de

src/backend/commands/vacuum.c

@@ -2077,7 +2077,7 @@ compute_parallel_delay(void)
 	VacuumCostBalanceLocal += VacuumCostBalance;
 
 	if ((shared_balance >= VacuumCostLimit) &&
-		(VacuumCostBalanceLocal > 0.5 * (VacuumCostLimit / nworkers)))
+		(VacuumCostBalanceLocal > 0.5 * ((double) VacuumCostLimit / nworkers)))
 	{
 		/* Compute sleep time based on the local cost balance */
 		msec = VacuumCostDelay * VacuumCostBalanceLocal / VacuumCostLimit;