Return 'stop' symbol in company-ycmd--prefix when cannot complete
Return 'stop' symbol in company-ycmd--prefix when cannot complete

The documentation states, that the prefix function should return 'stop'
when it should complete but cannot. Returning nil passes control to next
backend.

phst/emacs-ycmd,emacsmirror/ycmd,abingham/emacs-ycmd,abingham/emacs-ycmd,phst/emacs-ycmd,phst/emacs-ycmd,emacsmirror/ycmd,emacsmirror/ycmd,emacsmirror/ycmd,emacsmirror/ycmd,ptrv/emacs-ycmd,ptrv/emacs-ycmd,abingham/emacs-ycmd,ptrv/emacs-ycmd,phst/emacs-ycmd,ptrv/emacs-ycmd,ptrv/emacs-ycmd,phst/emacs-ycmd,emacsmirror/ycmd,ptrv/emacs-ycmd,abingham/emacs-ycmd,phst/emacs-ycmd,abingham/emacs-ycmd,abingham/emacs-ycmd