Null-terminate Collection.toArray destination only in JVM

In other platforms the elements following the collection elements
should not be changed.

As a part of efforts to stabilize Native stdlib.

libraries/stdlib/src/kotlin/collections/ArrayDeque.kt

@@ -541,12 +541,9 @@ public class ArrayDeque<E> : AbstractMutableList<E> {
             elementData.copyInto(dest, destinationOffset = 0, startIndex = head, endIndex = elementData.size)
             elementData.copyInto(dest, destinationOffset = elementData.size - head, startIndex = 0, endIndex = tail)
         }
-        if (dest.size > size) {
-            dest[size] = null // null-terminate
-        }
 
         @Suppress("UNCHECKED_CAST")
-        return dest as Array<T>
+        return terminateCollectionToArray(size, dest) as Array<T>
     }
 
     @Suppress("NOTHING_TO_OVERRIDE")

libraries/stdlib/src/kotlin/collections/Collections.kt

@@ -479,3 +479,9 @@ internal fun throwIndexOverflow() { throw ArithmeticException("Index overflow ha
 @SinceKotlin("1.3")
 internal fun throwCountOverflow() { throw ArithmeticException("Count overflow has happened.") }
 
+/**
+ * In JVM if the size of [array] is bigger than [collectionSize], sets `array[collectionSize] = null`.
+ * In other platforms does nothing.
+ * Returns the given [array].
+ */
+internal expect fun <T> terminateCollectionToArray(collectionSize: Int, array: Array<T>): Array<T>

libraries/stdlib/src/kotlin/collections/SlidingWindow.kt

@@ -146,9 +146,8 @@ private class RingBuffer<T>(private val buffer: Array<Any?>, filledSize: Int) :
             widx++
             idx++
         }
-        if (result.size > this.size) result[this.size] = null
 
-        return result as Array<T>
+        return terminateCollectionToArray(size, result) as Array<T>
     }
 
     override fun toArray(): Array<Any?> {