Backspace is ignored at the beginning of a list item if the previous list item is on a lower level

Here are the steps to reproduce:

• type this in wiki editor an then switch to wysiwyg:

  * a
  ** b
  * c
  

  or you can make the same list from the wysiwyg, it doesn't matter

• place the caret before the "c", at the beginning of the last list item
• press Backspace many times: nothing happens