Make StopProfiling resilient to calls in unusual moments.

src/cpu-profiler.cc

Index: src/cpu-profiler.cc
diff --git a/src/cpu-profiler.cc b/src/cpu-profiler.cc
index b8bff262f4289ff6d698ae00dd91c22b96dbeed4..d1230340e694c374a78642a489bb0216769e4238 100644
--- a/src/cpu-profiler.cc
+++ b/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;
 }