http://bugs.winehq.org/show_bug.cgi?id=22733
--- Comment #1 from Vincent Povirk madewokherd@gmail.com 2010-05-26 14:32:23 --- Seems like the call to get_xrender_format in XRender_AlphaBlend is failing, and we end up dereferencing a NULL pointer.
I don't know what the proper fix is, but you could probably work around it by running the rest of the special case only if fmt != NULL.