tiny and obviously correct (tm) fix for 0.12

[Justus Winter]

I've been going through the bindings lately and have some stuff in the
pipeline that has to wait until 0.12 is out, but I've noticed two
small and obvious errors I made when I annotated all the wrapped
libnotmuch functions.

The following patch addresses this problem and since it's a really
small change I'd like to see this in 0.12. What do you think?

Justus