Function split_body references variable before assignment

[Sebastian Pipping]

Hi!

Function split_body in `lxml/html/_diffcommand.py` may reference variables `pre`, `html`, and/or `post` prior to assignment at https://github.com/lxml/lxml/blob/master/src/lxml/html/_diffcommand.py#L82. This is how to reproduce the bug from the shell:

```
# python2 -m lxml.html.diff /dev/null{,}
Traceback (most recent call last):
  File "/usr/lib64/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib64/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/lib64/python2.7/site-packages/lxml/html/diff.py", line 880, in <module>
    _diffcommand.main()
  File "/usr/lib64/python2.7/site-packages/lxml/html/_diffcommand.py", line 43, in main
    body1 = split_body(input1)[1]
  File "/usr/lib64/python2.7/site-packages/lxml/html/_diffcommand.py", line 82, in split_body
    return pre, html, post
UnboundLocalError: local variable 'pre' referenced before assignment
```

Best, Sebastian

[scoder (scoder)]

https://github.com/lxml/lxml/commit/9ccbce92e706a1d28f3f98ce06f46f579910ff2a

[Sebastian Pipping (sebastian-pipping)]

Variable `post` has the same problem. That seems unfixed.

[Sebastian Pipping (sebastian-pipping)]

Edit: I misread "pre = post = ''" as "pre = ''", nevermind.