fix cancelAllFunctions
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

floxard/folly,brunomorishita/folly,facebook/folly,mqeizi/folly,brunomorishita/folly,charsyam/folly,floxard/folly,brunomorishita/folly,Orvid/folly,rklabs/folly,facebook/folly,brunomorishita/folly,facebook/folly,floxard/folly,charsyam/folly,rklabs/folly,Orvid/folly,charsyam/folly,facebook/folly,mqeizi/folly,floxard/folly,charsyam/folly,Orvid/folly,rklabs/folly,mqeizi/folly,mqeizi/folly,facebook/folly,floxard/folly,rklabs/folly,rklabs/folly,Orvid/folly,brunomorishita/folly,mqeizi/folly,Orvid/folly