Don't unconditionally cast uCont in ScopeCoroutine cast to CoroutineStackFrame

Fixes #2386

Author: willbuck

kotlinx-coroutines-core/common/src/internal/Scopes.kt

@@ -16,7 +16,7 @@ internal open class ScopeCoroutine<in T>(
     context: CoroutineContext,
     @JvmField val uCont: Continuation<T> // unintercepted continuation
 ) : AbstractCoroutine<T>(context, true), CoroutineStackFrame {
-    final override val callerFrame: CoroutineStackFrame? get() = uCont as CoroutineStackFrame?
+    final override val callerFrame: CoroutineStackFrame? get() = uCont as? CoroutineStackFrame
     final override fun getStackTraceElement(): StackTraceElement? = null
     final override val isScopedCoroutine: Boolean get() = true