AP_RCProtocol: add error check for ppm input

libraries/AP_RCProtocol/AP_RCProtocol_PPMSum.cpp

@@ -22,6 +22,11 @@
  */
 void AP_RCProtocol_PPMSum::process_pulse(uint32_t width_s0, uint32_t width_s1)
 {
+    if (width_s0 == 0 || width_s1 == 0) {
+        //invalid data: reset frame
+        ppm_state._channel_counter = -1;
+        return;
+    }
     uint32_t width_usec = width_s0 + width_s1;
     if (width_usec >= 2700) {
         // a long pulse indicates the end of a frame. Reset the