app/vlinsert/syslog: fix embedded newline in TCP syslog corrupting the next message

[func25]

Fixes #1365

A syslog message sent over TCP with an embedded newline could corrupt the next message. This is a known limitation of octet stuffing (see RFC 6587 §3.4.2). VictoriaLogs cannot avoid splitting such a message, but it should not let the leftover corrupt the next message and should handle it gracefully, as it already does for other malformed input.

Fixed by peeking the first byte to detect the framing method and reading each octet-stuffing frame up to its LF, so a frame can no longer leak into the next one. Also replaces the goto with a plain loop and drops the redundant prefix scan/copy, so the path does fewer reads per message.

+, syslog ingestion is best-effort to cover many weird cases of old syslog, so I don't think warning logs are needed here.

In the reported case, before

_msg: "Apr 29 17:43:00 ..."
_msg: ")\n<11>Apr 29 17:43:00 ..." # corrupted

after

_msg: "Apr 29 17:43:00 ..."
_msg: ")"  # bogus fragment isolated
_msg: "Apr 29 17:43:00 ..." # correctly parsed

[cubic-dev-ai]

1 issue found across 1 file (changes from recent commits).

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