logd: strip out empty trailing iovec

If the last buffer has zero length, strip it out of the iovec
issued to SocketClient::sendDatav().

Test: gTest liblog-unit-tests, logd-unit-tests, logcat-unit-tests
Bug: 36497967
Change-Id: I8fc585bbec63402d0e818ff4c620fdd7edcc38dc
1 file changed