hb_graphite2_cluster_t::advance can apparently be negative

...as seen with HarfBuzz used by LibreOffice, with `instdir/program/soffice
--headless --convert-to pdf` of doc/abi6073-2.doc from the LibreOffice crash-
testing corpus when run under UBSan,

> hb-graphite2.cc:361:15: runtime error: -1024 is outside the range of representable values of type 'unsigned int'
>  #0 in _hb_graphite2_shape at workdir/UnpackedTarball/harfbuzz/src/hb-graphite2.cc:361:15
>  #1 in _hb_shape_plan_execute_internal(hb_shape_plan_t*, hb_font_t*, hb_buffer_t*, hb_feature_t const*, unsigned int) at workdir/UnpackedTarball/harfbuzz/src/./hb-shaper-list.hh:38:1
>  #2 in hb_shape_plan_execute at workdir/UnpackedTarball/harfbuzz/src/hb-shape-plan.cc:453:14
>  #3 in hb_shape_full at workdir/UnpackedTarball/harfbuzz/src/hb-shape.cc:139:19
>  #4 in GenericSalLayout::LayoutText(ImplLayoutArgs&, SalLayoutGlyphsImpl const*) at vcl/source/gdi/CommonSalLayout.cxx:495:23
>  #5 in OutputDevice::getFallbackLayout(LogicalFontInstance*, int, ImplLayoutArgs&, SalLayoutGlyphs const*) const at vcl/source/outdev/font.cxx:1232:21
>  #6 in OutputDevice::ImplGlyphFallbackLayout(std::unique_ptr<SalLayout, std::default_delete<SalLayout> >, ImplLayoutArgs&, SalLayoutGlyphs const*) const at vcl/source/outdev/font.cxx:1300:48
>  #7 in OutputDevice::ImplLayout(rtl::OUString const&, int, int, Point const&, long, long const*, SalLayoutFlags, vcl::TextLayoutCache const*, SalLayoutGlyphs const*) const at vcl/source/outdev/text.cxx:1332:22
>  #8 in lcl_CreateLayout(SwTextGlyphsKey const&, __gnu_debug::_Safe_iterator<std::_Rb_tree_iterator<std::pair<SwTextGlyphsKey const, SwTextGlyphsData> >, std::__debug::map<SwTextGlyphsKey, SwTextGlyphsData, std::less<SwTextGlyphsKey>, std::allocator<std::pair<SwTextGlyphsKey const, SwTextGlyphsData> > >, std::bidirectional_iterator_tag>) at sw/source/core/txtnode/fntcache.cxx:233:33
>  #9 in SwFntObj::GetCachedSalLayoutGlyphs(SwTextGlyphsKey const&) at sw/source/core/txtnode/fntcache.cxx:257:12
>  #10 in SwFont::GetTextBreak(SwDrawTextInfo const&, long) at sw/source/core/txtnode/fntcache.cxx:2551:58
>  #11 in SwTextSizeInfo::GetTextBreak(long, o3tl::strong_int<int, Tag_TextFrameIndex>, unsigned short, vcl::TextLayoutCache const*) const at sw/source/core/text/inftxt.cxx:450:20
>  #12 in SwTextGuess::Guess(SwTextPortion const&, SwTextFormatInfo&, unsigned short) at sw/source/core/text/guess.cxx:205:26
>  #13 in SwTextPortion::Format_(SwTextFormatInfo&) at sw/source/core/text/portxt.cxx:305:32
>  #14 in SwTextPortion::Format(SwTextFormatInfo&) at sw/source/core/text/portxt.cxx:456:12
>  #15 in SwLineLayout::Format(SwTextFormatInfo&) at sw/source/core/text/porlay.cxx:260:31

(where in frame #4 GenericSalLayout::LayoutText, pHbBuffer->props.direction is
HB_DIRECTION_RTL, in case that is relevant).

It is unclear to me whether it is sufficient to only change
hb_graphite2_cluster_t::advance from signed to unsigned int, as there are other
unsigned int variables (like curradv in _hb_graphite2_shape) whose value depend
on hb_graphite2_cluster_t::advance, and which thus might also become negative.
But unlike the float -> unsigned int conversion that UBSan warned about here
(where gr_slot_origin_X() and xscale are float), those are signed int ->
unsigned int conversions that do not cause undefined behavior.  At least, with
this change, the above --convert-to pdf and a full `make check screenshot`
succeeded for me under without further UBSan warnings.

(For the version of HarfBuzz optionally built as part of the LibreOffice build,
this has been addressed with
<https://git.libreoffice.org/core/+/6e53e03f752c2f85283c4d47efaaf0683299783c%5E!/>
"external/harfbuzz: hb_graphite2_cluster_t::advance can apparently be
negative.")

src/hb-graphite2.cc

@@ -223,7 +223,7 @@ struct hb_graphite2_cluster_t {
   unsigned int base_glyph;
   unsigned int num_glyphs;
   unsigned int cluster;
-  unsigned int advance;
+  int advance;
 };
 
 hb_bool_t