Ensure we don't leak the ClassLoader in the backtrace of TrackRecord.BOTTOM (#10839)

Motivation:

We need to ensure we override fillInStacktrace() when we store exceptions in static fields to not leak the Classloader in the backtrace.

Came up in https://github.com/netty/netty/pull/10691#issuecomment-738331186. Thanks to @amir-shalem for notice this one.

Modifications:

- Add overrides of fillInStracktrace in TrackRecord.BOTTOM

Result:

Related fix to https://github.com/netty/netty/pull/10686

common/src/main/java/io/netty/util/ResourceLeakDetector.java

@@ -570,10 +570,20 @@ public class ResourceLeakDetector<T> {
         } while (!excludedMethods.compareAndSet(oldMethods, newMethods));
     }
 
-    private static final class TraceRecord extends Throwable {
+    private static class TraceRecord extends Throwable {
         private static final long serialVersionUID = 6065153674892850720L;
 
-        private static final TraceRecord BOTTOM = new TraceRecord();
+        private static final TraceRecord BOTTOM = new TraceRecord() {
+            private static final long serialVersionUID = 7396077602074694571L;
+
+            // Override fillInStackTrace() so we not populate the backtrace via a native call and so leak the
+            // Classloader.
+            // See https://github.com/netty/netty/pull/10691
+            @Override
+            public Throwable fillInStackTrace() {
+                return this;
+            }
+        };
 
         private final String hintString;
         private final TraceRecord next;