Fix performance issue with non-matching rules when no callback is provided.
Fix performance issue with non-matching rules when no callback is provided.

As pointed out by Arnim Rupp (@2d4d) in #172, the fix in #175 was not complete as there's was no performance gain when the callback was not provided, which is the most common case.

plusvic/yara-python,VirusTotal/yara-python,wxsBSD/yara-python,VirusTotal/yara-python,wxsBSD/yara-python,plusvic/yara-python