[LayoutNG] Fix crash in NGLineBreaker::Rewind()

This patch fixes a crash in NGLineBreaker::Rewind(), when
floats cause a call to HandleOverflow() without any
NGInlineItemResult.

Occurred in slack.com.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I1997b719c371141a53327fb81c8731177b0f9be0
Reviewed-on: https://chromium-review.googlesource.com/1129599Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573596}

third_party/blink/renderer/core/layout/ng/inline/ng_line_breaker.cc

@@ -1055,7 +1055,8 @@ NGLineBreaker::LineBreakState NGLineBreaker::HandleOverflow(
   if (break_anywhere_if_overflow_ && !override_break_anywhere_) {
     override_break_anywhere_ = true;
     break_iterator_.SetBreakType(LineBreakType::kBreakCharacter);
-    Rewind(line_info, 0);
+    if (!item_results->IsEmpty())
+      Rewind(line_info, 0);
     return LineBreakState::kLeading;
   }