[d3d] Memcpy segfault

[Robert Lunnon]

A reference application I use fails with a segfault in d3d, Its called Little 
Fighter 2

It fails after a call to BLT where both the source and dest are NULL and 
delivers this trace. I suspect the memcpy at line 1251 in surface_gdi.c then 
fails because sbuf or dbuf are invalid or NULL. I'm not very familiar with 
this code so I'd welcome views on the correct fix, (Other than just checking 
dbuf and sbuf for NULL)

trace:ddraw:IDirectDrawSurfaceImpl_Blt (7ff33bd8)->(0,0,0,1000400,7fd2fb20)
trace:d3d_surface:IWineGDISurfaceImpl_Blt (7ff33cb8)->(0,0,0,1000400,7fd2fb20)
trace:d3d_surface:IWineGDISurfaceImpl_LockRect (7ff33cb8) : rect at 0 
flags(00000000), output lockedRect at 7fd2fa28, memory at 7d6c0000
trace:d3d_surface:IWineD3DSurfaceImpl_GetPitch (7ff33cb8)
trace:d3d_surface:IWineD3DSurfaceImpl_GetPitch (7ff33cb8) Returning 2384
trace:d3d_surface:IWineGDISurfaceImpl_LockRect Locked Rect (7ff33d20) = l 0, t 
0, r 794, b 550
trace:d3d_surface:IWineGDISurfaceImpl_LockRect returning memory at 7d6c0000, 
pitch(2384)