XMLInputArchive SIGSEV segfault

[andrewkcorcoran]

The below code generates a segfault in the XMLInputArchive - we should probably be detecting xml which doesn't match expectations and throwing an exception.

#include <sstream>
#include "cereal/archives/xml.hpp"

int main()
{
    auto stream = std::istringstream{R"(
  <cereal>
  </cereal>
)"};
      int i;
      cereal::XMLInputArchive{stream}(i);
      return 0;
}