New unwind-protect byte-compiler warning

[Mattias Engdegård]

On master, the byte-compiler now warns about `unwind-protect` forms lacking 
actual unwinding code, which is very often a mistake. An Emacs build currently 
yields about 30 warnings of this kind.

I'm mostly going to wait for those with good knowledge about the code causing 
each warning to deal with it. Tell me if I can be of further help. There's no 
strong urgency.

This will eventually leave some warnings claimed by no-one for me to take on, 
which will be done in a conservative way, maybe after a week.