Added prohibiting adding of nullability in alternative signatures.

compiler/testData/readJavaBinaryClass/kotlinSignature/error/AddingNullability.java

@@ -0,0 +1,10 @@
+package test;
+
+import jet.runtime.typeinfo.KotlinSignature;
+
+public class AddingNullability {
+    @KotlinSignature("fun foo() : Int?")
+    public int foo() {
+        throw new UnsupportedOperationException();
+    }
+}

compiler/testData/readJavaBinaryClass/kotlinSignature/error/AddingNullability.kt

@@ -0,0 +1,9 @@
+package test
+
+import java.util.*
+
+public open class AddingNullability : Object() {
+    public open fun foo() : Int {
+        throw UnsupportedOperationException()
+    }
+}

compiler/testData/readJavaBinaryClass/kotlinSignature/error/AddingNullability.txt

@@ -0,0 +1,6 @@
+namespace test
+
+public open class test.AddingNullability : java.lang.Object {
+    public final /*constructor*/ fun <init>(): test.AddingNullability
+    public open fun foo(): jet.Int
+}