fix IndexError when min_num_of_chars is set to 0

Fixes #307. Error is thrown when g:ycm_min_num_of_chars_for_completion
option is set to 0 user tries to delete an identifier which starts at
the beginning of the line.
Merge pull request #308 from JazzCore/fix_307

fix IndexError when min_num_of_chars is set to 0