Avoid ThreadLocal memory leak in JoinPointImpl

according to https://rules.sonarsource.com/java/tag/leak/RSPEC-5164/.

Fixes #288.

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

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

@@ -163,10 +163,16 @@ public final String toLongString() {
 		if (arcs == null) {
 			arcs = new InheritableThreadLocalAroundClosureStack();
 		}
-		if (arc==null) {
-			this.arcs.get().pop();
+		Stack<AroundClosure> closureStack = arcs.get();
+		if (arc == null) {
+			closureStack.pop();
+			if (closureStack.empty()) {
+				// Avoid ThreadLocal memory leak according to https://rules.sonarsource.com/java/tag/leak/RSPEC-5164/.
+				// Should fix https://github.com/eclipse-aspectj/aspectj/issues/288.
+				arcs.remove();
+			}
 		} else {
-			this.arcs.get().push(arc);
+			closureStack.push(arc);
 		}
  	}