Update social_graph.cpp (#271)
Update social_graph.cpp (#271)

* Update social_graph.cpp

* fix crash when profile changed event while user no longer exists in graph

* Update social_graph.cpp

+ check nullptr

claytonv/xbox-live-api,claytonv/xbox-live-api,jicailiu/xbox-live-api,jasonsandlin/xbox-live-api,jasonsandlin/xbox-live-api,jasonsandlin/xbox-live-api,claytonv/xbox-live-api,jicailiu/xbox-live-api,jicailiu/xbox-live-api,jicailiu/xbox-live-api,jasonsandlin/xbox-live-api,claytonv/xbox-live-api,jasonsandlin/xbox-live-api,jicailiu/xbox-live-api,claytonv/xbox-live-api