fix flatCallTracer onexit out of bound

Author: amsanghi

eth/tracers/native/call_flat.go

@@ -195,6 +195,9 @@ func (t *flatCallTracer) OnExit(depth int, output []byte, gasUsed uint64, err er
 	if t.config.IncludePrecompiles {
 		return
 	}
+	if len(t.tracer.callstack[len(t.tracer.callstack)-1].Calls) > 0 {
+		return
+	}
 	var (
 		// call has been nested in parent
 		parent = t.tracer.callstack[len(t.tracer.callstack)-1]