Add clearBFCs method and clear BFCs after self-layout of scroller node.

Author: qianyuan.wqy

ios/sdk/WeexSDK/Sources/Component/WXScrollerComponent.mm

@@ -1152,6 +1152,11 @@ - (void)_layoutPlatform
             _flexCssNode->calculateLayout(renderPageSize);
             _flexCssNode->setParent(parent, _flexCssNode);
 
+            /* We must clear BFCs becuase we have set parent of _flexCSSNode to nullptr and
+             manually called its calculateLayout method. This will cause a non-bfc layout node
+             to have items in its BFCs vector. Later, a wild pointer may cause crash. */
+            _flexCssNode->clearBFCs();
+            
             // set origin and size back
             _flexCssNode->rewriteLayoutResult(left, top, width, height);
         }

weex_core/Source/core/layout/layout.h

@@ -731,6 +731,10 @@ namespace WeexCore {
 
       markDirty();
     }
+      
+    inline void clearBFCs() {
+        BFCs.clear();
+    }
 
     inline void addChildAt(WXCoreLayoutNode* const child, Index index) {
       mChildList.insert(mChildList.begin() + index, child);