Prior to JDK-8350441 the VM would not have started if we received a discontiguous heap reservation with all reservations smaller than the inital heap capacity. Now we crash because `ZPartition::prime` does not take this into account.

However in contrast to the page cache, the mapped cache makes it trivial to support this scenario. So I propose fixing `ZPartition::prime` to handle any discontiguous heap reservation.

Can be provoked in a debug build by using ZForceDiscontiguousHeapReservations > 16
`java -XX:+UseZGC -XX:ZForceDiscontiguousHeapReservations=17 -Xmx128m -Xms128m --version`