Transpose with an empty line crashes bpython #548

⊙  bpython                                                                                                                                                                                                                                                                                                                                                Julian@Macnetic
bpython version 0.15.dev155 on top of Python 2.7.10 /usr/local/opt/python/bin/python2.7
>>>
Traceback (most recent call last):
  File "/usr/local/bin/bpython", line 9, in <module>
    load_entry_point('bpython==0.15.dev155', 'console_scripts', 'bpython')()
  File "/usr/local/lib/python2.7/site-packages/bpython/curtsies.py", line 88, in main
    interactive=(not exec_args))
  File "/usr/local/lib/python2.7/site-packages/bpython/curtsies.py", line 189, in mainloop
    process_event(e)
  File "/usr/local/lib/python2.7/site-packages/bpython/curtsies.py", line 147, in process_event
    repl.process_event(e)
  File "/usr/local/lib/python2.7/site-packages/bpython/curtsiesfrontend/repl.py", line 524, in process_event
    return self.process_key_event(e)
  File "/usr/local/lib/python2.7/site-packages/bpython/curtsiesfrontend/repl.py", line 626, in process_key_event
    cut_buffer=self.cut_buffer)
  File "/usr/local/lib/python2.7/site-packages/bpython/curtsiesfrontend/manual_readline.py", line 67, in call
    return func(**args)
  File "/usr/local/lib/python2.7/site-packages/bpython/curtsiesfrontend/manual_readline.py", line 286, in transpose_character_before_cursor
    line[cursor_offset - 1] +
IndexError: string index out of range