Fix race condition segfault in JSSEngineReferenceImpl cleanup

The finalizer thread was experiencing segfaults during PR.Shutdown()
calls due to a race condition where multiple threads could execute
cleanup operations simultaneously on the same object.

Root cause:
- cleanup(), closeInbound(), closeOutbound(), and tryCleanup()
  methods were not synchronized
- Multiple threads could simultaneously check and modify the boolean
  flags (closed_fd, is_inbound_closed, is_outbound_closed)
- One thread could close/free ssl_fd while another thread was still
  using it in PR.Shutdown(), causing SIGSEGV in NSS memcpy

Changes:
- Add synchronized modifier to closeInbound(), closeOutbound(),
  cleanup(), and tryCleanup() methods
- The synchronized keyword provides both mutual exclusion (only one
  thread can execute these methods at a time) and memory visibility
  (changes to fields are visible to other threads)

This prevents concurrent cleanup operations from corrupting the native
PRFileDesc pointer and eliminates the segfault during finalization.

Assisted-by: Claude Sonnet 4.5

Author: edewata

base/src/main/java/org/mozilla/jss/ssl/javax/JSSEngineReferenceImpl.java

@@ -736,7 +736,7 @@ public void beginHandshake() throws SSLException {
     }
 
     @Override
-    public void closeInbound() {
+    public synchronized void closeInbound() {
         debug("JSSEngine: closeInbound()");
 
         if (!is_inbound_closed && ssl_fd != null && !closed_fd) {
@@ -750,7 +750,7 @@ public void closeInbound() {
     }
 
     @Override
-    public void closeOutbound() {
+    public synchronized void closeOutbound() {
         debug("JSSEngine: closeOutbound()");
 
         if (!is_outbound_closed && ssl_fd != null && !closed_fd) {
@@ -1675,7 +1675,7 @@ public SSLEngineResult wrap(ByteBuffer[] srcs, int offset, int length, ByteBuffe
      * connection.
      */
     @Override
-    public void tryCleanup() {
+    public synchronized void tryCleanup() {
         debug("JSSEngine: tryCleanup()");
         if (is_inbound_closed && is_outbound_closed) {
             // throw new RuntimeException("Probably shouldn't be here!");
@@ -1688,7 +1688,7 @@ public void tryCleanup() {
      * data streams if still open.
      */
     @Override
-    public void cleanup() {
+    public synchronized void cleanup() {
         debug("JSSEngine: cleanup()");
 
         if (!is_inbound_closed) {