Check pTmpFrm->GetType(), not nFrmType, for FRM_FLY

...as nFrmType need not match pTmpFrm->GetType() exactly, but can e.g. be
USHORT_MAX when GetFrmOfModify is called from SwFrmFmt::FindLayoutRect
(sw/source/core/layout/atrfrm.cxx).  It is probably good luck that the two
comparisons against FAR_AWAY made it too unlikely for the if's true branch to
ever be taken for a pTmpFrm that is not actually a SwFlyFrm.  Found the bug when
running CppunitTest_sw_ooxmlexport2 under -fsanitize=undefined, where

  ((SwFlyFrm*)pTmpFrm)->GetAnchorFrm()

was called for a pTmpFrm of type SwTabFrm.

Change-Id: Ia886a809bdb82488c3f410be66cf827b3f723c55