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) ^~~~~~~~~