underflow bug

[Mooophy]

test code:

include

include "stack_by_2queues.hpp"

int main()
{
ch10::stack_by_2queues stk(3);

for(int i = 0; i != 3; ++i)
    stk.push(i);

for(int i = 0; i != 3; ++i)
    std::cout << stk.pop() << std::endl;

return 0;

}

generated:
ch10: ../ch10/queue.hpp:73: T ch10::queue<int, std::vector<int, std::allocator > >::dequeue() [T = int, C = std::vector<int, std::allocator >]: Assertion `!empty() && "underflow"' failed.