Fix 'Not possible to change sign ...' issue

When user remove the first line of the file, vim-gitgutter tried to
replace the sign of the line 0 and the following error would caused.

    Error detected while processing function
    gitgutter#process_buffer..sign#update_sings..sign#upsert_new_gitgutter_signs:
    line    13:
    E885: Not possible to change sign GitGutterLineRemoved

While Vim does not have line 0, this commit simply ignore the sign
substitution when the specified line_number is 0.