gdiplus: return a NULL bitmap when GdipCreateBitmapFromScan0 fails

[Vincent Povirk]

Synthmaker (bug 15425) apparently does not check whether the call failed before disposing the image. Disposing the bogus image we give it causes a crash, but disposing NULL does not.

This gets the program to run slightly longer before crashing.

Perhaps the call should not be failing in this case, but we also should not give the address of a freed bitmap.
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