fix cancelAllFunctions

Summary: as titled. When I tried re-adding all functions after cancelAllFunctions, it throws error. This is because it doesn't reset currentFunction to null.

Reviewed By: jjs0

Differential Revision: D3209237

fb-gh-sync-id: 5b6e2d967fc3c0986320d23b8d61eb1bfbff8940
fbshipit-source-id: 5b6e2d967fc3c0986320d23b8d61eb1bfbff8940

folly/experimental/FunctionScheduler.cpp

@@ -223,6 +223,7 @@ void FunctionScheduler::cancelFunction(const std::unique_lock<std::mutex>& l,
 void FunctionScheduler::cancelAllFunctions() {
   std::unique_lock<std::mutex> l(mutex_);
   functions_.clear();
+  currentFunction_ = nullptr;
 }
 
 bool FunctionScheduler::resetFunctionTimer(StringPiece nameID) {