IndexError when the candidate is bad

[arnegoeteyn]

When using the Struct::function() syntax deoplete-rust throws the following error.

[deoplete] Traceback (most recent call last):                                                                                                                                
  File "/home/aggoetey/.local/share/nvim/plugged/deoplete.nvim/rplugin/python3/deoplete/child.py", line 220, in _gather_results                                              
    ctx['candidates'] = source.gather_candidates(ctx)                                                                                                                        
  File "/home/aggoetey/.local/share/nvim/plugged/deoplete-rust/rplugin/python3/deoplete/sources/rust.py", line 61, in gather_candidates                                      
    'kind': tokens[4],                                                                                                                                                       
IndexError: list index out of range                                                                                                                                          
Error from rust: IndexError('list index out of range').  Use :messages / see above for error details. 

Adding a simple check in the rust.py file to make sure that len(tokens) > 5 stops this crash from happening and makes it function as expected.