fix build on clang++ 3.8
this is the clang version which is shipped with ubuntu 16.10. it says:
default initialization of an object of const type 'const
>' (aka 'const
>') without a user-provided
I don't know if that error is correct or not, but initializing the
object uppon construction should not hurt...
I was pretty sure it would be default-initialised, but not so much anymore.