Coverity is worried that we check "service" at the end of these test
functions, since it doesn't see any way to reach the cleanup code
without having first dereferenced the variable.

Removing the check would be unwise in this case: instead we add a
tt_assert check before using "service" so that coverity thinks that
the check is doing something useful.

Bugfix on 0.3.2.1-alpha.