fix memory leak from exceptions

[EricTheMagician]

fix #436
Not sure if there's a cleaner way, but this works.

[eriknw]

Thanks! Looks reasonable to me. We may want to look more closely and add a couple tests for when sigspec (as it's called in the code) is passed to other functions while it's None.

[EricTheMagician]

@eriknw
Since it's such a simple fix, and that you know the codebase much more than I do, can I leave you to write the tests for these?

From what I had seen, there shouldn't be any issue, but I wouldn't know what to test beyond the existing tests.

[eriknw]

LGTM, thanks @thejinx0r and @akhilman! We're past due for a release, so I'll get this up soon.