Cherry-pick to 5.3: Fix handling of empty strings in UTF16BytesToString.

[andrewkroh]

Cherry-pick of PR #3705 to 5.3 branch. Original message:

Hi,

I encountered a bug while using winlogbeat on Windows Server 2003.
Some events had errors in them:

{'@timestamp': '2017-03-01T08:53:18.000Z',
 '@version': '1',
 'beat': {'hostname': '...', 'name': '..', 'version': '5.2.1'},
 'computer_name': '...',
 'event_id': 540,
 'host': '...',
 'level': 'Audit Success',
 'log_name': 'Security',
 'message_error': 'Slice must have an even length (length=141)',
 'record_number': '1209',
 'source_name': 'Security',
 'tags': ['beats_input_raw_event'],
 'type': 'eventlogging',
 'user': {'...'}}

It seems to be a bug in the UTF16BytesToString function.
The function does not detect empty strings correctly (due to an off-by-one check on the return value of indexNullTerminator), and it creates a misaligned offset in the buffer.

I added a test and fixed it.

For completeness, this is an example of a 'bad' record I had (encoded in hex):
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