Fix stack trace printing in d8

src/d8.cc

Index: src/d8.cc
diff --git a/src/d8.cc b/src/d8.cc
index 06499ef6dd39efd2c45962a907277f3013b4a6a0..a56d029f5a5fb2fedb3fb568578ca9b90f53bada 100644
--- a/src/d8.cc
+++ b/src/d8.cc
@@ -917,7 +917,8 @@ void Shell::ReportException(Isolate* isolate, v8::TryCatch* try_catch) {
     printf("\n");
     Local<Value> stack_trace_string;
     if (try_catch->StackTrace(isolate->GetCurrentContext())
-            .ToLocal(&stack_trace_string)) {
+            .ToLocal(&stack_trace_string) &&
+        stack_trace_string->IsString()) {
       v8::String::Utf8Value stack_trace(
           Local<String>::Cast(stack_trace_string));
       printf("%s\n", ToCString(stack_trace));