bug#15735: 24.3.50; `find-library' completion bug

[Drew Adams <drew.adams@oracle.com>]

emacs -Q

I add directory`ess-5.3.10' to my `load-path'.  That dir has a
subdirectory `lisp', which contains libraries whose names begin with
`ess', such as `ess.el'.  (But subdir `lisp' is not in `load-path'.)

M-x find-library ess TAB

completes to `ess-5.3.10', which it should not.  There is no library in
my `load-path' that has that prefix.

`RET' with the completion `ess-5.3.10' raises an error (naturally):

Debugger entered--Lisp error: (error "Can't find library ess-5.3.10/")
  signal(error ("Can't find library ess-5.3.10/"))
  error("Can't find library %s" "ess-5.3.10/")
  find-library-name("ess-5.3.10/")
  (find-file-noselect (find-library-name library))
  (let ((buf (find-file-noselect (find-library-name library)))) (condition-case nil (switch-to-buffer buf) (error (pop-to-buffer buf))))
  find-library("ess-5.3.10/")
  call-interactively(find-library record-it)
...

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-10-19 on LEG570
Bzr revision: 114715 rgm@gnu.org-20131019023520-s8mwtib7xcx9e05w
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking 'CFLAGS=-O0 -g3' CPPFLAGS=-DGLYPH_DEBUG=1'