Screenshot: fix crash in file panel

When you edit a folder name in Screenshot's file panel and hit the OK
button, it crashes in the BRefFilter. For MediaConverter, a workaround
was added until Tracker is fixed, see
https://cgit.haiku-os.org/haiku/tree/src/apps/mediaconverter/MediaConverterWindow.cpp#n61

Applying the same workaround here.

src/apps/screenshot/ScreenshotWindow.cpp

@@ -99,7 +99,12 @@ public:
 	virtual bool Filter(const entry_ref* ref, BNode* node,
 		struct stat_beos* stat, const char* filetype)
 	{
-		return node->IsDirectory();
+		// ToDo: Fix this properly in Tracker
+		// If you create a folder, then rename it, node is NULL.
+		// The BeBook says: "Note that the function is never sent an
+		// abstract entry, so the node, st, and filetype arguments will
+		// always be valid."
+		return node == NULL ? false : node->IsDirectory();
 	}
 };