fixing reception of packets with DLC=0

examples/CANReceiver/CANReceiver.ino

@@ -20,7 +20,7 @@ void loop() {
   // try to parse packet
   int packetSize = CAN.parsePacket();
 
-  if (packetSize) {
+  if (packetSize >= 0) {
     // received a packet
     Serial.print("Received ");
 

src/CANController.cpp

@@ -109,7 +109,7 @@ int CANControllerClass::endPacket()
 
 int CANControllerClass::parsePacket()
 {
-  return 0;
+  return -1;
 }
 
 long CANControllerClass::packetId()

src/ESP32SJA1000.cpp

@@ -219,7 +219,7 @@ int ESP32SJA1000Class::parsePacket()
 {
   if ((readRegister(REG_SR) & 0x01) != 0x01) {
     // no packet
-    return 0;
+    return -1;
   }
 
   _rxExtended = (readRegister(REG_SFF) & 0x80) ? true : false;

src/MCP2515.cpp

@@ -225,7 +225,7 @@ int MCP2515Class::parsePacket()
     _rxExtended = false;
     _rxRtr = false;
     _rxLength = 0;
-    return 0;
+    return -1;
   }
 
   _rxExtended = (readRegister(REG_RXBnSIDL(n)) & FLAG_IDE) ? true : false;