Workaround some Windows OpenGL drivers mishandling wglMakeCurrent().

You should be able to do wglMakeCurrent(NULL, NULL), according to MSDN, since
the first parameter is ignored if the second is NULL, but this causes problems
on some drivers, so make sure we always have a valid HDC in the first
parameter (and exit early if we already have no current context in the
NULL, NULL case).