Fix clang static analyzer warning
Fix clang static analyzer warning

If previousView was nil here, we'd crash attempting to add nil to a mutable array. This is not a functional change. (This code path wasn't actually possible due to the assertions earlier in the method, but it can't hurt.)

imjerrybao/UIView-AutoLayout,smileyborg/UIView-AutoLayout,hwsyy/UIView-AutoLayout,13983441921/UIView-AutoLayout,i-miss-you/UIView-AutoLayout,CanYouHelpMe/UIView-AutoLayout,cogddo/UIView-AutoLayout,HelloWilliam/UIView-AutoLayout,lijie121210/UIView-AutoLayout,hao-hua/UIView-AutoLayout,shenyuan000/UIView-AutoLayout,jianyelijun/UIView-AutoLayout,hugo-/UIView-AutoLayout