Fix compiler warning:

json_value.cpp: In copy constructor ‘Json::Value::CZString::CZString(const Json::Value::CZString&)’:
json_value.cpp:179:26: warning: enumeral and non-enumeral type in conditional expression [enabled by default]

I tested, and the patch should fix the warning.
But the following modifies have no effects.

Value::CZString::CZString(const CZString& other)
    : cstr_(static_cast<DuplicationPolicy>(other.index_) != noDuplication && other.cstr_ != 0
                ? duplicateStringValue(other.cstr_)
                : other.cstr_),
      index_(other.cstr_
                 ? (static_cast<DuplicationPolicy>(other.index_) == noDuplication ? noDuplication : duplicate)
                 : other.index_) {}