fixes for regexp field processors

[David Bremner]

This series fixes two bugs introduces in notmuch 0.24

1) queries of the form  'from:foo*' and 'subject:bar*' no longer work (the * is just silently dropped)
2) queries of the form 'from:""' and 'subject:""' crashes with an uncaught exception.

Unless there objections, I'll probably roll these patches (along with
perhaps the two memory fixes recently posted) into a bugfix release
0.24.1. Tentative release date early next week.

d