F-4364: override releaseNativeStruct in Hmac to serialize native free

src/main/java/com/wolfssl/wolfcrypt/Hmac.java

@@ -212,6 +212,18 @@ public class Hmac extends NativeStruct {
         }
     }
 
+    /**
+     * Release native Hmac structure.
+     * Synchronized on this object so a concurrent release cannot free
+     * the native struct while another synchronized method is using it.
+     */
+    @Override
+    public synchronized void releaseNativeStruct() {
+        synchronized (pointerLock) {
+            super.releaseNativeStruct();
+        }
+    }
+
     /**
      * Perform HMAC update operation
      *