Clarify the usage of last_error in YR_MEMORY_BLOCK_ITERATOR.
Clarify the usage of last_error in YR_MEMORY_BLOCK_ITERATOR.

VirusTotal/yara,pombredanne/yara,VirusTotal/yara,hillu/yara,hillu/yara,VirusTotal/yara,wxsBSD/yara,wxsBSD/yara,wxsBSD/yara,pombredanne/yara,wxsBSD/yara,wxsBSD/yara,hillu/yara,hillu/yara,pombredanne/yara,VirusTotal/yara,pombredanne/yara,VirusTotal/yara,hillu/yara,pombredanne/yara