Fix stepping in when paused in async functions.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
diff --git a/runtime/vm/debugger.cc b/runtime/vm/debugger.cc
index 3516af5ef6b57d2d4d8dcba69da41731af856ec4..cbb528e255a4d9c65836fb0a060482c673536c94 100644
--- a/runtime/vm/debugger.cc
+++ b/runtime/vm/debugger.cc
@@ -3279,8 +3279,9 @@ void Debugger::SetAsyncSteppingFramePointer() {
   if (!FLAG_async_debugger) {
     return;
   }
-  if (stack_trace_->FrameAt(0)->function().IsAsyncClosure() ||
-      stack_trace_->FrameAt(0)->function().IsAsyncGenClosure()) {
+  if ((stack_trace_->Length()) > 0 &&
+      (stack_trace_->FrameAt(0)->function().IsAsyncClosure() ||
+       stack_trace_->FrameAt(0)->function().IsAsyncGenClosure())) {
     async_stepping_fp_ = stack_trace_->FrameAt(0)->fp();
   } else {
     async_stepping_fp_ = 0;