Add debug output to stderr when decode_hex() is called on invalid hex input

src/hush_utils.h

@@ -858,7 +858,7 @@ int32_t unhex(char c)
     int32_t hex;
     if ( (hex= _unhex(c)) < 0 )
     {
-        //printf("unhex: illegal hexchar.(%c)\n",c);
+        fprintf(stderr,"unhex: illegal hexchar.(%c)\n",c);
     }
     return(hex);
 }