Fix copypaste error in SpriteSheet2D load code. Closes #1951.

Source/Urho3D/Urho2D/SpriteSheet2D.cpp

@@ -155,7 +155,7 @@ bool SpriteSheet2D::EndLoadFromPListFile()
     if (!texture_)
     {
         URHO3D_LOGERROR("Could not load texture " + loadTextureName_);
-        loadXMLFile_.Reset();
+        loadPListFile_.Reset();
         loadTextureName_.Clear();
         return false;
     }
@@ -191,7 +191,7 @@ bool SpriteSheet2D::EndLoadFromPListFile()
         DefineSprite(name, rectangle, hotSpot, offset);
     }
 
-    loadXMLFile_.Reset();
+    loadPListFile_.Reset();
     loadTextureName_.Clear();
     return true;
 }