Protect against API abuse.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index fde3b5d05078a26a443d31ab13cf1c00cd9d283b..2d850f755af3afc3c7ca16f23d6bd64c3c7909ad 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -6291,12 +6291,16 @@ void V8::SetCaptureStackTraceForUncaughtExceptions(
 
 void V8::SetCounterFunction(CounterLookupCallback callback) {
   i::Isolate* isolate = i::Isolate::UncheckedCurrent();
+  // TODO(svenpanne) The Isolate should really be a parameter.
+  if (isolate == NULL) return;
   isolate->stats_table()->SetCounterFunction(callback);
 }
 
 
 void V8::SetCreateHistogramFunction(CreateHistogramCallback callback) {
   i::Isolate* isolate = i::Isolate::UncheckedCurrent();
+  // TODO(svenpanne) The Isolate should really be a parameter.
+  if (isolate == NULL) return;
   isolate->stats_table()->SetCreateHistogramFunction(callback);
   isolate->InitializeLoggingAndCounters();
   isolate->counters()->ResetHistograms();
@@ -6305,6 +6309,8 @@ void V8::SetCreateHistogramFunction(CreateHistogramCallback callback) {
 
 void V8::SetAddHistogramSampleFunction(AddHistogramSampleCallback callback) {
   i::Isolate* isolate = i::Isolate::UncheckedCurrent();
+  // TODO(svenpanne) The Isolate should really be a parameter.
+  if (isolate == NULL) return;
   isolate->stats_table()->
       SetAddHistogramSampleFunction(callback);
 }