Make weak reference work only on non-nullable types. (#1496)

runtime/src/main/kotlin/konan/ref/Weak.kt

@@ -21,13 +21,12 @@ package konan.ref
  * retrieve a strong reference to an object, or return null, if object was already destoyed by
  * the memory manager.
  */
-class WeakReference<T> {
+class WeakReference<T : Any> {
     /**
      * Creates a weak reference object pointing to an object. Weak reference doesn't prevent
      * removing object, and is nullified once object is collected.
      */
     constructor(referred: T) {
-        if (referred == null) throw Error("Weak reference to null?")
         pointer = getWeakReferenceCounter(referred)
     }
 
@@ -49,4 +48,4 @@ class WeakReference<T> {
  * Returns either reference to an object or null, if it was collected.
  */
 @Suppress("NON_PUBLIC_CALL_FROM_PUBLIC_INLINE")
-public inline fun <reified T> WeakReference<T>.get() = pointer?.get() as T?
+public inline fun <reified T : Any> WeakReference<T>.get() = pointer?.get() as T?