Fix Ender 3 V2 (DWIN) buffer overrun (#19268)

Marlin/src/lcd/dwin/dwin_lcd.cpp

 #include "dwin_lcd.h"
 #include <string.h> // for memset
 
-// Make sure DWIN_SendBuf is large enough to hold the largest
-// printed string plus the draw command and tail.
-uint8_t DWIN_SendBuf[11 + 24] = { 0xAA };
+// Make sure DWIN_SendBuf is large enough to hold the largest string plus draw command and tail.
+// Assume the narrowest (6 pixel) font and 2-byte gb2312-encoded characters.
+uint8_t DWIN_SendBuf[11 + DWIN_WIDTH / 6 * 2] = { 0xAA };
 uint8_t DWIN_BufTail[4] = { 0xCC, 0x33, 0xC3, 0x3C };
 uint8_t databuf[26] = { 0 };
 uint8_t receivedType;
 }
 
 inline void DWIN_String(size_t &i, char * const string) {
-  const size_t len = strlen(string);
+  const size_t len = _MIN(sizeof(DWIN_SendBuf) - i, strlen(string));
   memcpy(&DWIN_SendBuf[i+1], string, len);
   i += len;
 }