Fix missing unbox state in native runBlocking

kotlinx-coroutines-core/native/src/Builders.kt

@@ -73,7 +73,7 @@ private class BlockingCoroutine<T>(
             eventLoop?.decrementUseCount()
         }
         // now return result
-        val state = state
+        val state = state.unboxState()
         (state as? CompletedExceptionally)?.let { throw it.cause }
         state as T
     }

kotlinx-coroutines-core/native/test/RunBlockingTest.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2016-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines
+import kotlin.test.*
+
+class RunBlockingTest : TestBase() {
+
+    @Test
+    fun testIncompleteState() {
+        val handle = runBlocking {
+            coroutineContext[Job]!!.invokeOnCompletion { }
+        }
+
+        handle.dispose()
+    }
+}