Fix bug in adding or removing tag by region.

There was a bug in notmuch-search-{add,remove}-tag-region, which would
not behave correctly if the region went beyond the last message. Now,
instead of simply iterating to the last line of the region, these
functions will iterate to the minimum of the last line of the region
and the last possible line, i.e.

(- (line-number-at-pos (point-max)) 2)

Tested-by: Carl Worth <cworth@cworth.org> Note that the old, buggy
behavior included infinite loops of emacs lisp code, so the new
behavior is significantly better than that.