[ili9xxx] Guard against null buffer in display_() when allocation fails (#15786)

esphome/components/ili9xxx/ili9xxx_defines.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <cstdint>
+
 namespace esphome {
 namespace ili9xxx {
 

esphome/components/ili9xxx/ili9xxx_display.cpp

@@ -229,6 +229,10 @@ void ILI9XXXDisplay::update() {
 }
 
 void ILI9XXXDisplay::display_() {
+  // buffer may be null if allocation failed
+  if (this->buffer_ == nullptr) {
+    return;
+  }
   // check if something was displayed
   if ((this->x_high_ < this->x_low_) || (this->y_high_ < this->y_low_)) {
     return;