Don't save pointer to temporary

[ellert]

The c_str() of a temporary std::string object is not valid after the
lifetime of the object expires. Consider:

$ cat temp.cpp
int main() {
  const char* c1 = std::string("ABC").c_str();
  const char* c2 = std::string("DEF").c_str();
  std::cout << c1 << std::endl;
  std::cout << c2 << std::endl;
}

$ g++ -O2 -o temp temp.cpp
$ ./temp
DEF
DEF

The memory used by the first temporary std::string is reused by the
second one, since the lifetime of the first has expired. So when the
saved c_str() of the first string is written out, it writes the second
string.