Fix NPE that was encounter by debugger (will never happen in real code). (#8992)

Motivation:

We synchronize on the chunk.arena when produce the String returned by PoolSubpage.toString() which may raise a NPE when chunk == null. Chunk == null for the head of the linked-list and so a NPE may raised by a debugger. This NPE can never happen in real code tho as we never access toString() of the head.

Modifications:

Add null checks and so fix the possible NPE

Result:

No NPE when using a debugger and inspect the PooledByteBufAllocator.

buffer/src/main/java/io/netty/buffer/PoolSubpage.java

@@ -204,16 +204,24 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
         final int maxNumElems;
         final int numAvail;
         final int elemSize;
-        synchronized (chunk.arena) {
+        if (chunk == null) {
-            if (!this.doNotDestroy) {
+            // This is the head so there is no need to synchronize at all as these never change.
-                doNotDestroy = false;
+            doNotDestroy = true;
-                // Not used for creating the String.
+            maxNumElems = 0;
-                maxNumElems = numAvail = elemSize = -1;
+            numAvail = 0;
-            } else {
+            elemSize = -1;
-                doNotDestroy = true;
+        } else {
-                maxNumElems = this.maxNumElems;
+            synchronized (chunk.arena) {
-                numAvail = this.numAvail;
+                if (!this.doNotDestroy) {
-                elemSize = this.elemSize;
+                    doNotDestroy = false;
+                    // Not used for creating the String.
+                    maxNumElems = numAvail = elemSize = -1;
+                } else {
+                    doNotDestroy = true;
+                    maxNumElems = this.maxNumElems;
+                    numAvail = this.numAvail;
+                    elemSize = this.elemSize;
+                }
             }
         }
 
@@ -227,6 +235,11 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
 
     @Override
     public int maxNumElements() {
+        if (chunk == null) {
+            // It's the head.
+            return 0;
+        }
+
         synchronized (chunk.arena) {
             return maxNumElems;
         }
@@ -234,6 +247,11 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
 
     @Override
     public int numAvailable() {
+        if (chunk == null) {
+            // It's the head.
+            return 0;
+        }
+
         synchronized (chunk.arena) {
             return numAvail;
         }
@@ -241,6 +259,11 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
 
     @Override
     public int elementSize() {
+        if (chunk == null) {
+            // It's the head.
+            return -1;
+        }
+
         synchronized (chunk.arena) {
             return elemSize;
         }