resend: Fix potential NULL deref in IWineD3DSurfaceImpl_LockRect

[Chris]

This patch fixes an issue in the aforementioned function, that would cause a 
NULL deref if swapchain and targetSwapChain are NULL and iface == 
myDevice->renderTarget. This fixes a crash for me when trying to exit from 
the boat in the game Morrowind.

(ps: this was additionally checked and cleared by Stefan Dösinger)
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