Set m_graphicsBuffer to nullptr after delete[]

Author: PaulZC

src/qwiic_oled_custom.h

@@ -81,7 +81,10 @@ class QwOLEDCustom : public QwGrSSD1306 {
     ~QwOLEDCustom()
     {
         if (m_graphicsBuffer != nullptr)
+        {
             delete[] m_graphicsBuffer;
+            m_graphicsBuffer = nullptr;
+        }
     };
 
     // set up the specific device settings