runtime: correct the asprintf shim for Windows

Ensure that the string that is formed from the `asprintf` call is
null-terminated.  The `_vsnprintf` call will not null-terminate the
string if the length is not sufficient.

stdlib/public/runtime/ReflectionMirror.mm

@@ -56,6 +56,7 @@ int asprintf(char **strp, const char *fmt, ...) {
     return -1;
 
   length = _vsnprintf(*strp, length, fmt, argp1);
+  (*strp)[length] = '\0';
 
   va_end(argp0);
   va_end(argp1);