winex11: Don't trace a garbage value or read past end of caller's array in X11DRV_wglChoosePixelFormatARB().

We always have a swapchain here, even if there are no back buffers.

Depending on the location flags, the buffer or heap_memory should be
used.

This check is even more inconsistent between versions. Notice how the v2
interface is particularly broken because it never AddRef()'s the new iface.
This check also seems to be the original source for the questionable behaviour
in d3d_device_set_render_target() of storing the new iface before checking the
result of the wined3d_device_set_render_target(). (In particular,
SetRenderTargetTest() in tests/d3d.c tests for this behavious.)

This is where things become a bit of a mess, because the error checking is
inconsistent between versions.