Don't crash on stack overflow entering the debugger.

src/execution.cc

Index: src/execution.cc
diff --git a/src/execution.cc b/src/execution.cc
index ea8cc3725d781ec30e672147d970402dfee3b472..561897567378e90e0d6011334aaf3ffbd65648cb 100644
--- a/src/execution.cc
+++ b/src/execution.cc
@@ -826,6 +826,11 @@ Object* Execution::DebugBreakHelper() {
     return isolate->heap()->undefined_value();
   }
 
+  StackLimitCheck check(isolate);
+  if (check.HasOverflowed()) {
+    return isolate->heap()->undefined_value();
+  }
+
   {
     JavaScriptFrameIterator it(isolate);
     ASSERT(!it.done());
@@ -862,6 +867,11 @@ void Execution::ProcessDebugMessages(bool debug_command_only) {
   // Clear the debug command request flag.
   isolate->stack_guard()->Continue(DEBUGCOMMAND);
 
+  StackLimitCheck check(isolate);
+  if (check.HasOverflowed()) {
+    return;
+  }
+
   HandleScope scope(isolate);
   // Enter the debugger. Just continue if we fail to enter the debugger.
   EnterDebugger debugger;