notmuch.el: Fix bug from message with ':' in the From address.

Eric reported that a particular thread was non-functional in the
notmuch-search mode in the emacs client. It was easy enough to trace
the bug down to a broken regular expression (using ':' instead of
';'). The bug would be triggered by a message with ':' in the
From address.

This is something I hope to add to the test suite as soon as we have
support for testing the emacs interface there.