Fix crash in case of empty file in directory

[mutt0-ds]

I have added a simple check for the issue #214, that skips the regex check SKIP_PATTERN when there are no lines to analyze.

I think that the fact that there is an empty file in the directory should be flagged as en error, but i would like to hear your opinion first before working on it.

Thanks for your work :)

[Prikalel]

What about

if (err.lnum - 1 not in range(len(self.lines)) or                       
        SKIP_PATTERN(self.lines[err.lnum - 1])):                        
    return None

here?

[mutt0-ds]

I thought about that but with this code pylama doesn't raise the D100 issue when executed with an empty file like mentioned in the example, there must be two if blocks
errors.py:1:1 D100 Missing docstring in public module [pydocstyle]

One alternative could be

if err.lnum - 1 in range(len(self.lines):
    if SKIP_PATTERN(self.lines[err.lnum - 1]):                        
        return None

[klen]

@mutt0-ds Thank you!