I didn’t say but it was on linux and I did what the wiki says to compile the editor:
make UE4Editor UE4Game UnrealPak CrashReportClient ShaderCompileWorker UnrealLightmass
From the error message it’s in the third party jpeg-compressor library that is used by the editor.
Note: This is with a development version of clang 3.8 which is not yet released. I believe that with the current stable releases of clang this warning does not exist yet and the build succeeds (I haven’t tested it).
The code probably still relies on undefined behavior so fixing it wouldn’t be a bad thing anyway.