reproducible debian: build.sh: fix depwait case

without the `exit 0` the process would continue into back into the main
branch, ending up trying to run update_rbuildlog() twice and failing
because the rbuild has already been taken care of.

Also remove unneeded return statements (the program will exit before
they are reached).

Signed-off-by: Mattia Rizzolo <mattia@debian.org>