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();
+    }
+}