It is worth fixing although it should never occur, as ddraw will never call GetBackBuffer(it manages them on it's own), and apparantly windows d3d8 and d3d9 always create a back buffer, even if backbuffercount = 0 (This needs a test case and a fix in d3d8 and
Look at the swapchains demo... it recently started to crash with NULL backbuffer issues. This is a d3d9 demo. Also, it may be relevant to your swapchain linked list fixes, since last time I tried it the second window was not textured - haven't tested since then (because it started crashing).
=>1 0x7e9061db IWineD3DSwapChainImpl_GetBackBuffer+0x3b(iface=0x7fd88a00, iBackBuffer=0x0, Type=0x0, ppBackBuffer=0x7f9cfc04) [/wine-git/dlls/wined3d/swapchain.c:385] in wined3d (0x7e9061db)
http://www.codesampler.com/dx9src/dx9src_1.htm#dx9_swap_chains