sw: fix wrong assert for broken documents like abi11860-2.odt

... that have page anchor type without text:anchor-page-number, so the
import stores a temporary anchor position that MakeFrms() cleans up.

Change-Id: I440d3a138659933e54f671d87bc418d5ba1059fb

sw/source/core/layout/atrfrm.cxx

@@ -3516,7 +3516,10 @@ void CheckAnchoredFlyConsistency(SwDoc const& rDoc)
             SwFmtAnchor const& rAnchor((**it).GetAnchor(false));
             if (FLY_AT_PAGE == rAnchor.GetAnchorId())
             {
-                assert(!rAnchor.GetCntntAnchor());
+                assert(!rAnchor.GetCntntAnchor()
+                    // for invalid documents that lack text:anchor-page-number
+                    // it may have an anchor before MakeFrms() is called
+                    || (!SwIterator<SwFrm, SwFrmFmt>(**it).First()));
             }
             else
             {