Fix for a multithreading crash.

SimdXContext::_fileName was a reference to a static std::string.  Two threads,
which each have their own SimdXContext, could simultaneously try to assign a
new value to _fileName, meaning they would simultaneously try to assign a new
value to the same static string.  In this case the assignment operation in each
thread would attempt to free the storage for the old value and you would get
a "double free" error.