KT-313 Bug in substitutions in a function returning its type parameter T

idea/testData/checker/Variance.jet

@@ -18,8 +18,8 @@ fun foo(c: Consumer<Int>, p: Producer<Int>, u: Usual<Int>) {
 }
 
 //Arrays copy example
-class Array<T>(val length : Int) {
-    fun get(index : Int) : T { return null }
+class Array<T>(val length : Int, val t : T) {
+    fun get(index : Int) : T { return t }
     fun set(index : Int, value : T) { /* ... */ }
 }