fix leak in Java event TLBs

Author: roberttoyonaga

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jfr/JfrThreadLocal.java

@@ -123,7 +123,9 @@ public UnsignedWord getThreadLocalBufferSize() {
     }
 
     public void teardown() {
+        // At this point all native buffers should be freed already.
         getNativeBufferList().teardown();
+        // At this point Java buffers will be retired or freed.
         getJavaBufferList().teardown();
     }
 
@@ -161,10 +163,11 @@ public static void stopRecording(IsolateThread isolateThread, boolean freeJavaBu
         flushToGlobalMemoryAndFreeBuffer(nb);
 
         JfrBuffer jb = javaBuffer.get(isolateThread);
-        javaBuffer.set(isolateThread, Word.nullPointer());
         if (freeJavaBuffer) {
+            javaBuffer.set(isolateThread, Word.nullPointer());
             flushToGlobalMemoryAndFreeBuffer(jb);
         } else {
+            // Do not reset the thread local since we may need it to reinstate the buffer in the next recording.
             flushToGlobalMemoryAndRetireBuffer(jb);
         }