tdf#51850 sw: fix a nullpointer crash

lcl_PasteFlyOrDrawFormat() may return nullptr, causing testTdf155028 to
crash in older release branch.

(regression from commit d072b5e09e)

Change-Id: I68549648aef2c9d58919f4d26415834d0784eeef
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/192173
Reviewed-by: Michael Stahl <michael.stahl@collabora.com>
Tested-by: Jenkins

sw/source/core/frmedt/fecopy.cxx

@@ -1079,7 +1079,7 @@ SwFEShell::Paste(SwDoc& rClpDoc, bool const bNestedTable)
                 {
                     lcl_InitSelectFlyOrDrawFormat(pFlyFormat, *this, isSelect);
                 }
-                if (isSelect && inserted.size() == 1)
+                if (isSelect && inserted.size() == 1 && inserted.front())
                 {
                     if (inserted.front()->Which() == RES_FLYFRMFMT
                         && GetDoc()->GetNodes()[inserted.front()->GetContent().GetContentIdx()->GetIndex() + 1]->IsGrfNode())