in_syslog: fix null byte handling in message parsing

[niedbalski]

Description

Null bytes in syslog messages are incorrectly treated as message delimiters, causing message fragmentation and parser errors. This violates RFC 6587 section 3.4.1 which specifies that for octet-counting framing, only the calculated length determines message boundaries, and section 3.4.2 which specifies newline as the delimiter for non-transparent framing.

Current behaviour

When a syslog message contains a null byte, the parser treats it as a message boundary:

[ warn] [input:syslog:syslog.0] error parsing log message with parser 'syslog-rfc5424'
[debug] [input:syslog:syslog.0] unparsed log message:  byte

• Message gets split at the null byte position
• First part gets processed normally
• Second part fails to parse (invalid syslog format)
• Results in parser errors and log spam

Expected behaviour

Null bytes should be preserved in message content and not treated as delimiters:

• Complete message gets processed as single unit
• Null byte preserved in final parsed message
• No parser errors or message fragmentation

Changes

Modified syslog_prot_process() to skip null bytes when scanning for message boundaries, only treating newlines as delimiters per RFC 6587 section 3.4.2.

Fixes #2741