Allow yr_hash to receive zero-length strings.
Allow yr_hash to receive zero-length strings.

Fixes https://github.com/VirusTotal/yara-python/issues/93.

plusvic/yara,pombredanne/yara,plusvic/yara,cblichmann/yara,hillu/yara,plusvic/yara,hillu/yara,plusvic/yara,plusvic/yara,VirusTotal/yara,cblichmann/yara,wxsBSD/yara,wxsBSD/yara,hillu/yara,wxsBSD/yara,hillu/yara,pombredanne/yara,VirusTotal/yara,wxsBSD/yara,VirusTotal/yara,cblichmann/yara,pombredanne/yara,VirusTotal/yara,cblichmann/yara,pombredanne/yara,cblichmann/yara,hillu/yara,VirusTotal/yara,pombredanne/yara,wxsBSD/yara