fix(clipboard): update version regex pattern (neovim#21012)

Building tmux from source uses a 'next-' prefix, so account for that.
Also handle failures to match more gracefully.

runtime/autoload/provider/clipboard.vim

@@ -139,8 +139,8 @@ function! provider#clipboard#Executable() abort
  let s:paste['*'] = s:paste['+']
  return 'termux-clipboard'
  elseif !empty($TMUX) && executable('tmux')
- let [major, minor] = matchlist(systemlist(['tmux', '-V'])[0], 'tmux \(\d\+\)\.\(\d\+\)')[1:2]
- if major > 3 || (major == 3 && minor >= 2)
+ let ver = matchlist(systemlist(['tmux', '-V'])[0], '\vtmux %(next-)?(\d+)\.(\d+)')
+ if len(ver) >= 3 && (ver[1] > 3 || (ver[1] == 3 && ver[2] >= 2))
  let s:copy['+'] = ['tmux', 'load-buffer', '-w', '-']
  else
  let s:copy['+'] = ['tmux', 'load-buffer', '-']