Remove document listener from document on handleDispose()

UpdateDocumentListener is added to document but not always removed in
ProjectionViewer, regression introduced by commit
1848058

This fixes only one possible leak, where a single document was used in
ProjectionViewer, to unblock JDT UI tests.

Fixes eclipse-jdt/eclipse.jdt.ui#2532

Author: iloveeclipse

bundles/org.eclipse.jface.text/projection/org/eclipse/jface/text/source/projection/ProjectionViewer.java

@@ -1480,6 +1480,12 @@ private boolean willAutoExpand(Position position, int offset, int length) {
 	@Override
 	protected void handleDispose() {
 		fWasProjectionEnabled= false;
+		if (fUpdateDocumentListener != null) {
+			IDocument document= getDocument();
+			if (document != null) {
+				document.removeDocumentListener(fUpdateDocumentListener);
+			}
+		}
 		super.handleDispose();
 	}