Fixes assertion error: src/mutex.hpp:123

[revaultch]

The issue is sort o the same as here: #47
Assertion error, when destroing ctx while creating a socket from a different thread: ./src/mutex.hpp:123
Test program:
clang++ -std=c++1y -stdlib=libc++ test.cc libzmq/src/.libs/libzmq.a -I cppzmq -I libzmq/include -o unfixed && ./unfixed
Tested with 4-1 HEAD

#include <thread>
#include <iostream>
#include <memory>
#include "zmq.hpp"

int main() {
  for (int i=0; i< 100000; i++) {
    if (i%10==0)
      std::cout << "iteration " << i << std::endl;
    auto ctx = std::make_shared<zmq::context_t>();

    std::thread contextcloser([ctx] {
        ctx->close();
      });
    std::thread socketopener([ctx] {
        zmq::socket_t s(*ctx, ZMQ_DEALER);
      });
    socketopener.join();
    contextcloser.join();
  }
}