[KxSerialization] Fix NPE if sealed class has self-referencing property

Fixes Kotlin/kotlinx.serialization#2294

If the serializable class contains itself as a property, and its serializer is not an object (for example, for a sealed class), in this case there is a race in the initialization of the serializer and child serializers.

To avoid this the serialization class should not cache its own serializer as a child.

Merge-request: KT-MR-10019
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

plugins/kotlinx-serialization/testData/boxIr/caching.kt

@@ -0,0 +1,29 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+
+// Serializers for classes containing themselves as properties should be cached correctly
+@Serializable
+class Plain(val p: Plain? = null)
+
+@Serializable
+sealed class Sealed(val p: Sealed? = null)
+
+@Serializable
+open class Open(val p: Open? = null)
+
+@Serializable
+abstract class Abstract(val p: Abstract? = null)
+
+
+fun box(): String {
+    // A correctly cached class must be initialized correctly in order to exclude cyclic nesting of caches
+    Plain.serializer()
+    Sealed.serializer()
+    Open.serializer()
+    Abstract.serializer()
+
+    return "OK"
+}