Tracker: Fix memory leak in IconCache::SetIcon.

How often this would have occurred before thumbnail support I'm not sure,
but at least now that we have thumbnails, the icons are changed out
with not insignificant frequency, especially on read-only filesystems.

src/kits/tracker/IconCache.cpp

@@ -292,18 +292,24 @@ void
 IconCacheEntry::SetIcon(BBitmap* bitmap, IconDrawMode mode, icon_size size,
 	bool /*create*/)
 {
+	BBitmap** icon = NULL;
 	if (mode == kNormalIcon) {
 		if (size == B_MINI_ICON)
-			fMiniIcon = bitmap;
+			icon = &fMiniIcon;
 		else
-			fLargeIcon = bitmap;
+			icon = &fLargeIcon;
 	} else if (mode == kSelectedIcon) {
 		if (size == B_MINI_ICON)
-			fHighlightedMiniIcon = bitmap;
+			icon = &fHighlightedMiniIcon;
 		else
-			fHighlightedLargeIcon = bitmap;
+			icon = &fHighlightedLargeIcon;
-	} else
+	}
+	if (icon == NULL)
 		TRESPASS();
+
+	if ((*icon) != NULL)
+		delete *icon;
+	*icon = bitmap;
 }