Fix merging for maplikeSerializer when the map is not empty (by using…

… the actual size * 2). (#1712)

The index in the conceptual array is twice the amount of elements in the list. This way the
reading is consistent between single and multipass. Also decoders can thus use even/odd to
determine whether to read the key or value.

core/commonMain/src/kotlinx/serialization/internal/CollectionSerializers.kt

@@ -257,7 +257,7 @@ internal class LinkedHashMapSerializer<K, V>(
     override fun Map<K, V>.collectionSize(): Int = size
     override fun Map<K, V>.collectionIterator(): Iterator<Map.Entry<K, V>> = iterator()
     override fun builder(): LinkedHashMap<K, V> = LinkedHashMap()
-    override fun LinkedHashMap<K, V>.builderSize(): Int = size
+    override fun LinkedHashMap<K, V>.builderSize(): Int = size * 2
     override fun LinkedHashMap<K, V>.toResult(): Map<K, V> = this
     override fun Map<K, V>.toBuilder(): LinkedHashMap<K, V> = this as? LinkedHashMap<K, V> ?: LinkedHashMap(this)
     override fun LinkedHashMap<K, V>.checkCapacity(size: Int) {}
@@ -273,7 +273,7 @@ internal class HashMapSerializer<K, V>(
     override fun Map<K, V>.collectionSize(): Int = size
     override fun Map<K, V>.collectionIterator(): Iterator<Map.Entry<K, V>> = iterator()
     override fun builder(): HashMap<K, V> = HashMap()
-    override fun HashMap<K, V>.builderSize(): Int = size
+    override fun HashMap<K, V>.builderSize(): Int = size * 2
     override fun HashMap<K, V>.toResult(): Map<K, V> = this
     override fun Map<K, V>.toBuilder(): HashMap<K, V> = this as? HashMap<K, V> ?: HashMap(this)
     override fun HashMap<K, V>.checkCapacity(size: Int) {}