Fixed codegen crash on use of protected synthetic extension property

compiler/testData/codegen/boxAgainstJava/syntheticExtensions/_protected.kt

@@ -0,0 +1,11 @@
+package p
+
+import _protected
+
+fun box(): String {
+    return KotlinClass().ok()
+}
+
+class KotlinClass : _protected() {
+    fun ok() = ok
+}