bug#27818: 26.0.50; *occur* buffer misses one match

[Dani Moncayo]

Recipe from "emacs -Q":

1.  Create a new buffer and put there these four lines:
---begin-of-buffer---------
w1    # word 1
w2    # word 2
w3    # word 3
w4    # word 4
---end-of-buffer---------

2. Search for the following regexp:
"[^[:alnum:]]\(w2\|w3\)[^[:alnum:]]" (without the quotes)

3.  End the search and type "C-M-s M-p RET M-s o" to launch an *occur* buffer.

The search actually has 2 matches but the *occur* buffer shows only one of them:
---begin-of-buffer---------
1 match for "[^[:alnum:]]\(w2\|w3\)[^[:alnum:]]" in buffer: foo
      1:w1    # word 1
       :w2    # word 2
---end-of-buffer---------

I'm using a build made from a recent version of the master branch:

In GNU Emacs 26.0.50 (build 1, x86_64-unknown-cygwin)
 of 2017-07-24 built on ZVDES404
Repository revision: 6dc5d45c542a6f9cfbcf3e37d597c9e0efb3070d
Windowing system distributor 'Microsoft Corp.', version 6.3.9600

Configured using:
 'configure --with-mailutils --with-w32'

Configured features:
SOUND ACL GNUTLS ZLIB TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: C.ISO-8859-1
  locale-coding-system: iso-latin-1-unix

-- 
Dani Moncayo