Handle case with no detected checkbox

Author: harshad1

plugin/bullets.vim

@@ -558,8 +558,10 @@ endfun
 fun! s:set_checkbox(lnum, marker)
   let l:initpos = getpos('.')
   let l:pos = s:find_checkbox_position(a:lnum)
-  call s:replace_char_in_line(a:lnum, l:pos, a:marker)
-  call setpos('.', l:initpos)
+  if l:pos >= 0
+      call s:replace_char_in_line(a:lnum, l:pos, a:marker)
+      call setpos('.', l:initpos)
+  endif
 endfun
 
 fun! s:toggle_checkboxes_nested()