Fix default constructor of IntegralRangeInterator
In c++-14 all value-initilized forward iterators
should behave like the end-iterator of a hypothetic
empty container and furthermore should all compare equal.
Here, this would lead to undefined behaviour since
the value_
member is not initialized.
To avoid this we no initilize it by 0 which mimics
'past end of empty range'. With this patch we can check
IntegralRangeInterator
using the testConstIterator()
test suite.