Fix memory leak

drivers/display_st25dvdiscovery.h

@@ -216,7 +216,7 @@ public:
         /**
          * Default constructor, results in an invalid iterator.
          */
-        pixel_iterator(): pixelLeft(0) {}
+        pixel_iterator(): pixelLeft(0), wr(nullptr) {}
 
         /**
          * Set a pixel and move the pointer to the next one
@@ -275,6 +275,11 @@ public:
          */
         void invalidate() {}
 
+        ~pixel_iterator()
+        {
+            if(wr) delete wr;
+        }
+
     private:
 
         /**