Fix an off-by-one error in vmservice expression evaluation.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index 104d22c4bb448658f790e8c91288bf5047522100..d35f0e147f42af9a33dac52f31623cf8d6a56f6c 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -2306,7 +2306,7 @@ static bool EvaluateInFrame(Thread* thread, JSONStream* js) {
   }
   DebuggerStackTrace* stack = isolate->debugger()->StackTrace();
   intptr_t framePos = UIntParameter::Parse(js->LookupParam("frameIndex"));
-  if (framePos > stack->Length()) {
+  if (framePos >= stack->Length()) {
     PrintInvalidParamError(js, "frameIndex");
     return true;
   }