lib: fix possible assert() fail in zlog_fd()

lib: fix possible assert() fail in zlog_fd()

If the last message in a batched logging operation isn't printed due to
priority, this skips the code that flushes prepared messages through
writev() and can trigger the assert() at the end of zlog_fd().

Since any logmsg above info priority triggers a buffer flush, running
into this situation requires a log file target configured for info
priority, at least 1 message of info priority buffered, a debug message
buffered after that, and then a buffer flush (explicit or due to buffer
full).

I haven't seen this chain of events happen in the wild, but it needs
fixing anyway.

Signed-off-by: David Lamparter <equinox@diac24.net>