GL: fix segfault in GLGSRender::flip

[Megamouse]

It turns out that image_to_flip2 is nullptr in GLGSRender::flip, which then leads to a simple_array of size 2 and then images.map(FN...). It then tries to call xform with the dereferenced nullptr in simple_array::map.

So there's 3 potential bugs here:

1. deref nullptr in map
2. using a simple_array of size 2 even though image_to_flip2 is nullptr
3. having a nullptr in image_to_flip2 in the first place

I tried to check the iterator for nullptr, but it seems to enter the scope and call xform anyway.
So we can just make sure not to call map with a nullptr in the first place.

fixes #15361