Bug 383236 Fix missing null-check from previous patch for bug 382746 r=Enn sr=bz

layout/xul/base/src/nsSprocketLayout.cpp

@@ -452,6 +452,7 @@ nsSprocketLayout::Layout(nsIBox* aBox, nsBoxLayoutState& aState)
       {
         childComputedBoxSize = childComputedBoxSize->next;
         childBoxSize = childBoxSize->next;
+        if (child)
           child = child->GetNextBox();
         count++;
         x = nextX;