Make iparserdocstream more like std::istream

Make iparserdocstream more like std::istream

In C++98 std::istream does not use an operator bool(), but an operator
void*() instead, which prevents some unwanted conversions (this is one
possible implementation of the safe bool idiom).
In C++11 std::istream uses explicit operator bool, which prevents the unwanted
conversions using a new language feature.

This change does not have any effect on correct code, but prevents some
mistakes.