Synchronise JoinPointImpl methods dealing with Stack<AroundClosure>

If we do not synchronise, we will run into problems with nested
around-aspects in combination with proceeding asynchronously (in another
thread).

Fixes eclipse-aspectj#128.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

runtime/src/main/java/org/aspectj/runtime/reflect/JoinPointImpl.java

@@ -146,7 +146,7 @@ public final String toLongString() {
 		this.arc = arc;
 	}
 
- 	public void stack$AroundClosure(AroundClosure arc) {
+ 	public synchronized void stack$AroundClosure(AroundClosure arc) {
 		// If input parameter arc is null this is the 'unlink' call from AroundClosure
 		if (arcs == null) {
 			arcs = new Stack<>();
@@ -158,7 +158,7 @@ public final String toLongString() {
 		}
  	}
 
-	public Object proceed() throws Throwable {
+	public synchronized Object proceed() throws Throwable {
 		// when called from a before advice, but be a no-op
 		if (arcs == null) {
 			if (arc == null) {
@@ -171,7 +171,7 @@ public Object proceed() throws Throwable {
 		}
 	}
 
-	public Object proceed(Object[] adviceBindings) throws Throwable {
+	public synchronized Object proceed(Object[] adviceBindings) throws Throwable {
 		// when called from a before advice, but be a no-op
 		AroundClosure ac = null;
 		if (arcs == null) {