Fixed codegen crash on use of protected synthetic extension property

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

@@ -0,0 +1,15 @@
+package p
+
+import _protectedSetter
+
+fun box(): String {
+    return KotlinClass().ok()
+}
+
+class KotlinClass : _protectedSetter() {
+    fun ok(): String {
+        x = "o"
+        x += "k"
+        return x
+    }
+}