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

logd/LogBufferElement.cpp

@@ -235,7 +235,9 @@ log_time LogBufferElement::flushTo(SocketClient* reader, LogBuffer* parent,
     }
     iovec[1].iov_len = entry.len;
 
-    log_time retval = reader->sendDatav(iovec, 2) ? FLUSH_ERROR : mRealTime;
+    log_time retval = reader->sendDatav(iovec, 1 + (entry.len != 0))
+                          ? FLUSH_ERROR
+                          : mRealTime;
 
     if (buffer) free(buffer);