Avoid unhandled NPEs in FoldingReconcilerStrategy

The exception below is printed to standard error every time a generic
editor diff contribution is used in compare editor for target files. I
assume it never worked as designed with TargetPlatformFoldingReconciler,
so just add a NPE guard to prevent unhandled exceptions / printouts.

```
Exception in thread
"org.eclipse.pde.internal.genericeditor.target.extension.reconciler.folding.TargetPlatformFoldingReconciler"
java.lang.NullPointerException: Cannot invoke
"org.eclipse.jface.text.source.projection.ProjectionViewer.getProjectionAnnotationModel()"
because "this.projectionViewer" is null
	at org.eclipse.pde.internal.genericeditor.target.extension.reconciler.folding.FoldingReconcilerStrategy.initialReconcile(FoldingReconcilerStrategy.java:63)
	at org.eclipse.jface.text.reconciler.Reconciler.initialProcess(Reconciler.java:223)
	at org.eclipse.jface.text.reconciler.AbstractReconciler$BackgroundThread.run(AbstractReconciler.java:177)
```

See eclipse-platform/eclipse.platform.ui#1798

ui/org.eclipse.pde.genericeditor.extension/src/org/eclipse/pde/internal/genericeditor/target/extension/reconciler/folding/FoldingReconcilerStrategy.java

@@ -60,6 +60,9 @@ public void reconcile(IRegion partition) {
 
 	@Override
 	public void initialReconcile() {
+		if (projectionViewer == null || document == null) {
+			return;
+		}
 		ProjectionAnnotationModel projectionAnnotationModel = projectionViewer.getProjectionAnnotationModel();
 		if (document.get().equals(oldDocument) || projectionAnnotationModel == null)
 			return;