Tracker: Removing the Desktop Background Image don't clear a pointer

fBackgroundImage wasn't being cleared when removing the desktop
background image because the loading process wasn't ignoring
loading failures (as is the "no background for this workspace" setting).

fBackgroundImage not being NULL was causing crashes mainly in
ScreenChanged(), where it was trying to re-center a non-existant
background.  Was causing #7376 and maybe others.

src/kits/tracker/BackgroundImage.cpp

@@ -93,6 +93,7 @@ BackgroundImage::GetBackgroundImage(const BNode *node, bool isDesktop)
 			bitmap = BTranslationUtils::GetBitmap(path);
 			if (!bitmap) {
 				PRINT(("failed to load background bitmap from path\n"));
+				continue;
 			}
 		} else
 			break;