[LayoutNG] Add null check for return value of ContainingNGBlockFlow().

The function ContainingNGBlockFlow() might return a nullptr.
So add a null check before dereferencing the pointer returned
by it.

Bug: None
Change-Id: Ib4c3f84166654ec7af3bc187190714f2c3ab2bee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505352Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Prashant Nevase <prashant.n@samsung.com>
Commit-Queue: Prashant Nevase <prashant.n@samsung.com>
Cr-Commit-Position: refs/heads/master@{#638089}

third_party/blink/renderer/core/paint/ng/ng_paint_fragment.cc

@@ -83,9 +83,9 @@ NGLogicalRect ExpandedSelectionRectForSoftLineBreakIfNeeded(
   // this fragment is at the end of line.
   if (selection_status.line_break == SelectSoftLineBreak::kNotSelected)
     return rect;
-  if (paint_fragment.GetLayoutObject()
-          ->ContainingNGBlockFlow()
-          ->ShouldTruncateOverflowingText())
+  LayoutBlockFlow* layout_block_flow =
+      paint_fragment.GetLayoutObject()->ContainingNGBlockFlow();
+  if (layout_block_flow && layout_block_flow->ShouldTruncateOverflowingText())
     return rect;
   // Copy from InlineTextBoxPainter::PaintSelection.
   const LayoutUnit space_width(paint_fragment.Style().GetFont().SpaceWidth());