valgrind: SfxBaseModel's pImpl of fallback TargetDocument for paste leaks

e.g. repeatedly pasting a metafile into writer and deleting it.

==13369== 342 (232 direct, 110 indirect) bytes in 1 blocks are definitely lost in loss record 19,396 of 22,170
==13369==    at 0x4A06965: operator new(unsigned long) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==13369==    by 0x686D72E: SfxBaseModel::SfxBaseModel(SfxObjectShell*) (sfxbasemodel.cxx:518)
==13369==    by 0x288CA9C1: SvxUnoDrawingModel::SvxUnoDrawingModel(SdrModel*) (unomod.cxx:255)
==13369==    by 0x28961055: SvxDrawingLayerImport(SdrModel*, com::sun::star::uno::Reference<com::sun::star::io::XInputStream> const&, com::sun::star::uno::Reference<com::sun::star::lang::XComponent> const&, char const*) (xmlexport.cxx:164)
==13369==    by 0x28960FA2: SvxDrawingLayerImport(SdrModel*, com::sun::star::uno::Reference<com::sun::star::io::XInputStream> const&, com::sun::star::uno::Reference<com::sun::star::lang::XComponent> const&) (xmlexport.cxx:148)
==13369==    by 0x2896175E: SvxDrawingLayerImport(SdrModel*, com::sun::star::uno::Reference<com::sun::star::io::XInputStream> const&) (xmlexport.cxx:254)
==13369==    by 0x240B1002: SwFEShell::Paste(SvStream&, unsigned short, Point const*) (fecopy.cxx:1333)
==13369==    by 0x247B981A: SwTransferable::_PasteSdrFormat(TransferableDataHelper&, SwWrtShell&, unsigned short, Point const*, unsigned char, bool) (swdtflvr.cxx:2207)
==13369==    by 0x247B678E: SwTransferable::PasteData(TransferableDataHelper&, SwWrtShell&, unsigned short, unsigned long, unsigned short, bool, bool, Point const*, signed char, bool) (swdtflvr.cxx:1481)
==13369==    by 0x247B5AE4: SwTransferable::Paste(SwWrtShell&, TransferableDataHelper&) (swdtflvr.cxx:1150)
==13369==    by 0x2488FCCE: SwBaseShell::ExecClpbrd(SfxRequest&) (basesh.cxx:277)

     Reference< lang::XComponent > xTargetDocument( xComponent );
     if( !xTargetDocument.is() )
     {
>        xTargetDocument = new SvxUnoDrawingModel( pModel );
         pModel->setUnoModel( Reference< XInterface >::query( xTargetDocument ) );
     }

(cherry picked from commit fdca15fb864aca975533257d5ee83440992d8a7e)

Conflicts:
	sfx2/source/doc/sfxbasemodel.cxx

Change-Id: I3bd0fc983d9b6bdcdc91071a4ec3d2c584397ebf
Reviewed-on: https://gerrit.libreoffice.org/9384
Tested-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>