Crash when fetching clipboard data.

Crash when fetching clipboard data.

Seems like some unusual corner-case around HTML export.

	sw::DocumentContentOperationsManager::CopyImplImpl(SwPaM&, SwPosition&, bool, SwPaM*, bool) const
		/sw/source/core/doc/DocumentContentOperationsManager.cxx:5068
	sw::DocumentContentOperationsManager::CopyImpl(SwPaM&, SwPosition&, bool, SwPaM*, bool) const
		/sw/source/core/doc/DocumentContentOperationsManager.cxx:4609
	sw::DocumentContentOperationsManager::CopyRange(SwPaM&, SwPosition&, bool, bool, bool) const
		/sw/source/core/doc/DocumentContentOperationsManager.cxx:1936
	SwEditShell::CopySelToDoc(SwDoc*)
		/sw/inc/pam.hxx:193 (discriminator 2)
	SwFEShell::Copy(SwDoc*, rtl::OUString const*)
		/sw/source/core/frmedt/fecopy.cxx:224
	(anonymous namespace)::lclOverWriteDoc(SwWrtShell&, SwDoc&)
		/sw/source/uibase/dochdl/swdtflvr.cxx:413
	SwTransferable::GetData(com::sun::star::datatransfer::DataFlavor const&, rtl::OUString const&)
		/include/sfx2/objsh.hxx:866
	TransferableHelper::getTransferData2(com::sun::star::datatransfer::DataFlavor const&, rtl::OUString const&)
		/include/com/sun/star/uno/Type.h:121
	TransferableHelper::getTransferData(com::sun::star::datatransfer::DataFlavor const&)
		/include/rtl/ustring.hxx:438
	getFromTransferrable.isra.0
		/include/com/sun/star/uno/Any.hxx:151
	encodeImageAsHTML
		/desktop/source/lib/init.cxx:4382
	doc_getTextSelection

Change-Id: I1af52d827ebdc9bbc5278f56547d1b3fd1b87e7c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/125454
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>