Fix compilation warnings under cparser/firm

See mail at https://lists.torproject.org/pipermail/tor-dev/2012-October/004094.html

I'm making a ticket for this, so that I can apply it 0.2.3 if we decide we need to.

(Do we need to? I think that building warning-free under a new compiler we didn't try before is something not to do in a stable series at this point in our understanding of the word "stable", but then again this is a wafer-thin patch.)