SI-7080 improve boundary value checking for BitSet

When BitSet accepts a very large integer such as Int.MaxValue,
integer overflow possibly occurs in the calculation of boundary
value "nwords * WordLength". This faulty boundary condition
causes empty-iterator problem like following:

scala> import collection.mutable.BitSet
import collection.mutable.BitSet

scala> val x = BitSet(Int.MaxValue)
x: scala.collection.mutable.BitSet = BitSet()

scala> x.iterator
res0: Iterator[Int] = empty iterator