Bug 1575847: Do a racy early return from profiler_get_backtrace if the profiler isn't active. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D43069

--HG--
extra : moz-landing-system : lando

tools/profiler/core/platform.cpp

@@ -3928,6 +3928,11 @@ double profiler_time() {
 UniqueProfilerBacktrace profiler_get_backtrace() {
   MOZ_RELEASE_ASSERT(CorePS::Exists());
 
+  // Fast racy early return.
+  if (!profiler_is_active()) {
+    return nullptr;
+  }
+
   PSAutoLock lock(gPSMutex);
 
   if (!ActivePS::Exists(lock) || ActivePS::FeaturePrivacy(lock)) {