Make StopProfiling resilient to calls in unusual moments.

Review URL: http://codereview.chromium.org/1658005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4435 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/cpu-profiler.cc

@@ -253,14 +253,12 @@ void CpuProfiler::StartProfiling(String* title) {
 
 
 CpuProfile* CpuProfiler::StopProfiling(const char* title) {
-  ASSERT(singleton_ != NULL);
-  return singleton_->StopCollectingProfile(title);
+  return singleton_ != NULL ? singleton_->StopCollectingProfile(title) : NULL;
 }
 
 
 CpuProfile* CpuProfiler::StopProfiling(String* title) {
-  ASSERT(singleton_ != NULL);
-  return singleton_->StopCollectingProfile(title);
+  return singleton_ != NULL ? singleton_->StopCollectingProfile(title) : NULL;
 }