Fix an issue where app will crash if web view controller is deallocated before web view finishes loading.
Fix an issue where app will crash if web view controller is deallocated before web view finishes loading.

ch-beck/SVWebViewController,adonoho/SVWebViewController,HarrisLee/SVWebViewController,hoowang/SVWebViewController,Gaoxian/SVWebViewController,inbilin-inc/SVWebViewController,jianwoo/SVWebViewController,1yvT0s/SVWebViewController,bouralu89/SVWebViewController,TransitApp/SVWebViewController,benzhipeng/SVWebViewController,rtsbtx/SVWebViewController,9gag-open-source/SVWebViewController,mohsinalimat/SVWebViewController,hoanganh6491/SVWebViewController,keslcod/SVWebViewController,Kingside/SVWebViewController