Avoid potential negative array index access to cached text.

Avoid potential negative array index access to cached text.

    if ((nIndex != 0 || nLen != text.getLength()) && !skipGlyphSubsets)
	seems unlikely to protect us from this:

	/opt/rh/devtoolset-12/root/usr/include/c++/12/string_view:239: constexpr const std::basic_string_view<_CharT, _Traits>::value_type& std::basic_string_view<_CharT, _Traits>::operator[](size_type) const [with _CharT = char16_t; _Traits = std::char_traits<char16_t>; const_reference = const char16_t&; size_type = long unsigned int]: Assertion '__pos < this->_M_len' failed.

	coolwsd[16958] ... SIG   Fatal signal received: SIGABRT code: 18446744073709551610 for address: 0x7300004e16

	SalLayoutGlyphsCache::GetLayoutGlyphs(VclPtr<OutputDevice const>, rtl::OUString const&, int, int, long, vcl::text::TextLayoutCache const*)
		/home/collabora/jenkins/workspace/build_core_co-24.04_for_online_snapshot/vcl/source/gdi/impglyphitem.cxx:399
	GetTextArray(OutputDevice const&, rtl::OUString const&, KernArray&, int, int, bool, vcl::text::TextLayoutCache const*)
		/home/collabora/jenkins/workspace/build_core_co-24.04_for_online_snapshot/include/rtl/ref.hxx:128
	SwFntObj::GetTextSize(SwDrawTextInfo&)
		/home/collabora/jenkins/workspace/build_core_co-24.04_for_online_snapshot/sw/source/core/txtnode/fntcache.cxx:766
	SwSubFont::GetTextSize_(SwDrawTextInfo&)
		/home/collabora/jenkins/workspace/build_core_co-24.04_for_online_snapshot/sw/source/core/txtnode/swfont.cxx:1022
	SwTextSizeInfo::GetTextSize() const
		/home/collabora/jenkins/workspace/build_core_co-24.04_for_online_snapshot/sw/source/core/inc/swfont.hxx:314

	blind fix - but seeing a lot of these.

Change-Id: Icb6ca25e4b8c6ef8a5e5b89dfa01b56bb788378d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/168575
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Tested-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>