Fix a crash when we try to print ending stats but no tests were run.

This can happen if the server gets into a weird state (see the test
cases for reproducing issue #212).  We need to do a couple of checks
to make sure we're not dereferencing NULL pointers (yay C).

While here, also fix up a couple of related output glitches, where
in this case we can emit some invalid JSON (NaN values, such as what
you get if there's a division by zero, are not valid JSON).

Part of a fix in progress for #212.