Check current word length before invoking completion

This simple change adds a check of current word length before invoking the completion. If the length of current word is smaller than `g:ycm_min_num_of_chars_for_completion`, completion is not invoked. This prevents `User defined completion (^U^N^P) Pattern not found` vim error message to appear.

The error message is annoying and useless if `g:ycm_min_num_of_chars_for_completion` is set high.