TT#14008 use temporary variable to avoid stray compiler warning
The glib macro for g_atomic_pointer_get() uses typeof() with an
intermediate variable, resulting in a stray compiler warning if the
argument uses `const` as the intermediate variable also ends up being
const. Use an extra non-const intermediate variable to work around this.
closes#1270
Change-Id: I3bf1404240d3b8571aaf40c38b524f578e0fdbd9