Resolves: fdo#59183 Copy 4 or more slides then crash

regression from 17afe4cea7e01aef1e5270cc09f438bc6fde3211 which is totally
forgivable as its riddled with asserts that suggest there should be no
out-of-bounds accesses and there probably shouldn't and those queries are
possibly bugs. But double-checking 3-6 it is the case that non-existant pages
were queried for in that version too, so return NULL on out-of-bounds
like the original pre-stl conversion code did.

Change-Id: Ic918419b6cb76b083de6b6911dde9d6e00258324
diff --git a/svx/source/svdraw/svdmodel.cxx b/svx/source/svdraw/svdmodel.cxx
index aae7534..01abfb7 100644
--- a/svx/source/svdraw/svdmodel.cxx
+++ b/svx/source/svdraw/svdmodel.cxx
@@ -1996,13 +1996,13 @@ SvxNumType SdrModel::GetPageNumType() const
const SdrPage* SdrModel::GetPage(sal_uInt16 nPgNum) const
{
    DBG_ASSERT(nPgNum < maPages.size(), "SdrModel::GetPage: Access out of range (!)");
    return maPages[nPgNum];
    return nPgNum < maPages.size() ? maPages[nPgNum] : NULL;
}

SdrPage* SdrModel::GetPage(sal_uInt16 nPgNum)
{
    DBG_ASSERT(nPgNum < maPages.size(), "SdrModel::GetPage: Access out of range (!)");
    return maPages[nPgNum];
    return nPgNum < maPages.size() ? maPages[nPgNum] : NULL;
}

sal_uInt16 SdrModel::GetPageCount() const