CDC/Uart: fix the event handler to provide an early out if no data is read

Author: GrumpyOldPizza

cores/stm32l4/CDC.cpp

@@ -403,6 +403,10 @@ void CDC::EventCallback(uint32_t events)
 	    if (empty && _receiveCallback) {
 		(*_receiveCallback)(count);
 	    }
+
+	    if (!rx_size) {
+		break;
+	    }
 	}
     }
 

cores/stm32l4/Uart.cpp

@@ -393,6 +393,10 @@ void Uart::EventCallback(uint32_t events)
 	    if (empty && _receiveCallback) {
 		(*_receiveCallback)(count);
 	    }
+
+	    if (!rx_size) {
+		break;
+	    }
 	}
     }