prompt_blending.py raises an index out of bound exception

[autumnmotor]

Sometimes prompt_blending.py raises an index out of bound exception, I think this part should be fixed

       if c == REAL_MARK and stack[-1] == '{':
           p[i] = MARK

I think "stack[-1]" sometimes tries to access an invalid index and raises an exception.

   if len(stack) > 0:
       if c == REAL_MARK and stack[-1] == '{':
           p[i] = MARK