syslogd: Do not print bad data

[jfree]

Summary

If errno is zero, then buf is never filled and we're printing
uninitialized data. Print the msg buffer instead to mitigate this.

[markj]

Seems ok, but we might want to put a diagnostic in if the function is called with errno == 0, since that probably implies a bug in the caller. You could even just remove the errno != 0 check.

[jfree]

Update after rebase