S.BUS parsing improvements

src/lib/rc/sbus.c

@@ -214,23 +214,23 @@ sbus_input(int sbus_fd, uint16_t *values, uint16_t *num_values, bool *sbus_fails
 
 		/* if the read failed for any reason, just give up here */
 		if (ret < 1) {
-			continue;
+			break;
 		}
 
 		last_rx_time = now;
 
+		/*
+		 * Add bytes to the current frame
+		 */
+		partial_frame_count += ret;
+
 		/* if the first byte of the frame is not the start symbol, give up instantly */
-		if (frame[0] != SBUS_START_SYMBOL) {
+		if (partial_frame_count > 0 && (frame[0] != SBUS_START_SYMBOL)) {
 			sbus_frame_drops++;
 			partial_frame_count = 0;
 			continue;
 		}
 
-		/*
-		 * Add bytes to the current frame
-		 */
-		partial_frame_count += ret;
-
 		/*
 		 * If we don't have a full frame, return
 		 */