clockoverlay: fix bogus time display caused by previous commit

clockoverlay: fix bogus time display caused by previous commit

Fixes regression introduced by "clean-up" done as part of commit 98ebcb4.

dummy must live as long as use the return value of localtime_r() since
that's just a pointer to it, and by putting it inside the block we made
dummy go out of scope right after localtime_r() returned, which messed
up the time values since when we poked at the struct the contents might
already have been overwritten.

Fixes #722