[HWSerial] Replace assert() by Error_Handler()

Avoid to include "assert.h" which relies on printf().
This allow to save space:
Example for SerialEvent sketch for BluePill:
 - with assert:
Sketch uses 16692 bytes (25%) of program storage space. Maximum is 65536 bytes.
Global variables use 1092 bytes (5%) of dynamic memory, leaving 19388 bytes for local variables. Maximum is 20480 bytes.
 - without assert:
Sketch uses 12288 bytes (18%) of program storage space. Maximum is 65536 bytes.
Global variables use 984 bytes (4%) of dynamic memory, leaving 19496 bytes for local variables. Maximum is 20480 bytes.

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/HardwareSerial.cpp

@@ -26,7 +26,6 @@
 #include <stdio.h>
 #include <string.h>
 #include <inttypes.h>
-#include <assert.h>
 #include "Arduino.h"
 #include "HardwareSerial.h"
 
@@ -385,10 +384,9 @@ void HardwareSerial::begin(unsigned long baud, byte config)
       break;
     default:
 	case 0:
-      databits = 0;
+      Error_Handler();
       break;
   }
-  assert(databits!=0);
 
   uart_init(&_serial);
   uart_attach_rx_callback(&_serial, _rx_complete_irq);