Icon-O-Matic: Fix memory leak

I found leak_analyser.sh and gave it a try. Indeed, it did find many
leaks, including this one.

Change-Id: I7d36274f3f7772aa459c4bf1d30392e7b8cfa171
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6467
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

src/apps/icon-o-matic/MainWindow.cpp

@@ -145,6 +145,7 @@ MainWindow::~MainWindow()
 	// stuff is properly detached
 	delete fDocument;
 
+	delete fCurrentColor;
 	delete fMessageAfterSave;
 }