[Morse] Fix possible overflow

src/protocols/Morse/Morse.cpp

@@ -56,7 +56,7 @@ char MorseClient::decode(uint8_t symbol, uint8_t len) {
   }
 
   // nothing found
-  return(RADIOLIB_MORSE_UNSUPPORTED);
+  return(RADIOLIB_MORSE_UNKNOWN_SYMBOL);
 }
 
 #if !RADIOLIB_EXCLUDE_AFSK

src/protocols/Morse/Morse.h

@@ -14,6 +14,7 @@
 #define RADIOLIB_MORSE_INTER_SYMBOL                             0x00
 #define RADIOLIB_MORSE_CHAR_COMPLETE                            0x01
 #define RADIOLIB_MORSE_WORD_COMPLETE                            0x02
+#define RADIOLIB_MORSE_UNKNOWN_SYMBOL                           '*'
 
 // Morse character table: - using codes defined in ITU-R M.1677-1
 //                        - Morse code representation is saved LSb first, using additional bit as guard