gvalue: Use g_value_clear as clear function

This change allow leaving a scope before g_value_init() has been
called. This would happen if you do:

  {
    g_auto(GValue) value = G_VALUE_INIT;
  }

Or have a return statement (due to failure) before the part of
your code where you set this GValue.

https://bugzilla.gnome.org/show_bug.cgi?id=755766