cron::cron_next : check cronexpr initialized to avoid endless recursion

[AverageBeerEnjoyer]

This code produces segfault

#include "croncpp.h"
#include <chrono>

using namespace cron;
using namespace std;

int main() { 
    cronexpr expr;
    auto tp = chrono::system_clock::now();
    auto next = cron::cron_next(expr, tp);
}

due to endless recursion in

template <typename Traits>
static bool find_next(cronexpr const & cex,
                      std::tm& date,
                      size_t const dot)

I added an initialization check based on emptiness of cronexpr::expr, which is always not empty when cronexpr is initialized.