Fix possible crash copying malformed memcache value responses
Fix possible crash copying malformed memcache value responses

Closes #436

andyqzb/twemproxy,twitter/twemproxy,twitter/twemproxy,andyqzb/twemproxy,twitter/twemproxy,andyqzb/twemproxy,twitter/twemproxy,andyqzb/twemproxy