Agenda filter by any tag seems to be broken

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Is anyone else seeing this?

Go into a tags-type agenda, hit "/" to start filtering, then SPC for
"filter to any tag". That gives me this backtrace:

Debugger entered--Lisp error: (args-out-of-range "" 0 1)
  substring("" 0 1)
  (equal "{" (substring tag 0 1))
  (and (equal "{" (substring tag 0 1)) (equal "}" (substring tag -1)))

  org-agenda-filter-make-matcher-tag-exp(("+") 43)

Once ("+") is added to `org-agenda-tag-filter', all subsequent filtering
actions will fail, until you remove all filters with "/ /".

`org-agenda-filter-make-matcher-tag-exp' always removes the first
character of the tag-matcher string, and then operates on what's left.
In the case of "+", nothing's left!

Thanks,
Eric