`In-word expansion` not work for `g:UltiSnipsListSnippets` if prefixed by other chars

[ZSaberLv0]

Expected behavior:

Actual behavior:

Steps to reproduce

1. define a in-word snip, for example:

   snippet exists "exists check" im
   exists('${1}')
   endsnippet
   

2. input !exists and then:

   • use g:UltiSnipsExpandTrigger to trigger, works correctly
   • use g:UltiSnipsListSnippets to trigger, nothing happened
     • if remove the prefix !, g:UltiSnipsListSnippets would trigger correctly
     • if add a space after the !, i.e. ! exists, g:UltiSnipsListSnippets would trigger correctly

---

(I don't think the env matters, tested on Windows and MacOS, both happen, here's the env of my MacOS)

• Operating System: MacOS 10.14
• Vim Version: VIM - Vi IMproved 8.1 (2018 May 18, compiled Nov 23 2018 09:29:30) macOS version Included patches: 1-280
• UltiSnips Version: latest (9a01a928ce36620817774eef7084362041c74040)
• Python inside Vim: 3.6.7 (default, Oct 21 2018, 08:56:20) [GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.2)]
• Docker repo/vimrc:

  let g:UltiSnipsListSnippets = "<c-o>"
  let g:UltiSnipsExpandTrigger = "<c-u>"
  let g:UltiSnipsJumpForwardTrigger = "<c-o>"
  let g:UltiSnipsJumpBackwardTrigger = "<c-u>"
  let g:UltiSnipsRemoveSelectModeMappings = 0
  

[SirVer]

Confirmed at dc9d8f5e16a0a0727a47309e6c9e9ded26bc3e0e. Thanks for reporting!