WineD3D: Do not addref a NULL backbuffer

[Ivan Gyurdiev]

> 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