Fixed codegen crash on use of protected synthetic extension property

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

@@ -0,0 +1,3 @@
+public class _protected {
+    protected String getOk() { return "OK"; }
+}

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
+}

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

@@ -0,0 +1,6 @@
+public class _protectedSetter {
+    private String x = null;
+
+    public String getX() { return "OK"; }
+    protected void setX(String x) { this.x = x; }
+}

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
+    }
+}