fix a bug

bitsk · bitsk · commit 0c42ce6f2812 · 2019-04-12T19:31:59.000+08:00
diff --git a/cores/arduino/UARTClass.cpp b/cores/arduino/UARTClass.cpp
@@ -107,7 +107,7 @@ UARTClass::read(void)
   while(this->_buff->available()){
     return this->_buff->read_char();
   }
-  return 0;
+  return -1;
 }
 
 void
