Check for empty / Null files

[maksym3d]

The validator appears to throw an exception when the submitted file is null - since a null / non-existing file is not an infected file, I would suggest wrapping the initial seek(0) with a try/except:

# Check if the file is not empty / exists
try:
    file.seek(0)
except:
    return

[vstoykov]

Fixed in #19