Check if file is actually a file object (#32)
Check if file is actually a file object (#32)

If it is not a file like object, the function would return -1. This
would eventually lead to a segmention fault later on.
plusvic/yara-python,grrrrrrrrr/yara-python,wxsBSD/yara-python,plusvic/yara-python,grrrrrrrrr/yara-python,wxsBSD/yara-python,VirusTotal/yara-python,VirusTotal/yara-python