Fix NullPointerException in AppendOnlyLinkedArrayList.forEachWhile on a full last chunk (#8174)

* Fix NullPointerException in AppendOnlyLinkedArrayList.forEachWhile on a full last chunk

forEachWhile(S, BiPredicate) walks the linked chunks and, after iterating a
chunk, follows the next-chunk pointer stored at a[capacity]. When the list size
is exactly a multiple of the chunk capacity the last chunk is full but its
next-chunk pointer has not been allocated yet (it is null), so `a` becomes null
and the next iteration throws NullPointerException reading a[0].

Stop traversal when the next chunk is null. Adds a regression test that fails
before this change (NPE) and passes after it.

* Refactor forEachWhile to use lambda in test

Refactor test method to use lambda expression.

---------

Co-authored-by: David Karnok <akarnokd@gmail.com>

Author: Vasiliy Mikhailov

src/main/java/io/reactivex/rxjava4/internal/util/AppendOnlyLinkedArrayList.java

@@ -175,6 +175,9 @@ public <S> void forEachWhile(S state, BiPredicate<? super S, ? super T> consumer
                 }
             }
             a = (Object[])a[c];
+            if (a == null) {
+                return;
+            }
         }
     }
 }

src/test/java/io/reactivex/rxjava4/internal/util/MiscUtilTest.java

@@ -236,4 +236,23 @@ public void appendOnlyLinkedArrayListForEachWhileBiAll() throws Throwable {
     public void queueDrainHelperUtility() {
         TestHelper.checkUtilityClass(QueueDrainHelper.class);
     }
+
+    @Test
+    public void appendOnlyLinkedArrayListForEachWhileBiFullChunkNoNext() throws Throwable {
+        // Fill exactly capacity elements in one chunk; no next chunk exists (a[c] == null).
+        // The predicate returns false for all elements so the loop must traverse past the full chunk.
+        AppendOnlyLinkedArrayList<Integer> list = new AppendOnlyLinkedArrayList<>(2);
+
+        list.add(1);
+        list.add(2);
+
+        final List<Integer> out = new ArrayList<>();
+
+        list.forEachWhile(null, (state, value) -> {
+            out.add(value);
+            return false; // never terminate early
+        });
+
+        assertEquals(Arrays.asList(1, 2), out);
+    }
 }