[PATCH] mf/evr: Initialize variables which are checked conditionally.

[Biswapriyo Nath]

> Is there actually a compiler which complains about these?

Yes. With clang (llvm-mingw[1]). Although there are other
initialization warnings, this one is different because the objects are
also checked before releasing memory (the fearful goto failed;). The
warnings are like this:

../wine-source/dlls/mf/evr.c:2076:9: note: uninitialized use occurs here
    if (mixer)
        ^~~~~
../wine-source/dlls/mf/evr.c:2079:9: note: uninitialized use occurs here
    if (presenter)
        ^~~~~~~~~

[1]: https://github.com/mstorsjo/llvm-mingw