Fixes a static analyzer warning about unused variables for disabled logging

Fixes the following warnings that were shown for (e.g. release) builds with logging disabled.
```
GCDWebServer/Core/GCDWebServer.m:845:13: warning: Value stored to 'message' during its initialization is never read
GCDWebServer/Core/GCDWebServer.m:854:13: warning: Value stored to 'message' during its initialization is never read
GCDWebServer/Core/GCDWebServer.m:863:13: warning: Value stored to 'message' during its initialization is never read
```