Add regression test for bug #19

I committed the fix for bug #19 without a test because I didn't know
how our test worked yet; here's a test.

A somewhat more desirable way to do this would be to commit the
test *first*, marked as "incomplete" with `g_test_incomplete()` (an
expected failure).  However, `g_test_incomplete()` does not handle
segfaults!  There's no way to mark a segfaulting test as an "expected
segfault".  So if you want to verify that this test reveals the bug,
you'll need to `git show thiscommit | patch -p1` or something, in a
tree that doesn't have the fix applied.  Or you can comment out the
fix, I guess.