Fix the -Werror=strict-overflow correctly

...from 2bf2dee04c1d698e586316c2bd18a3e34df4be77 "Werror=strict-overflow."  The
warning (generated e.g. by --disable-debug --disable-dbgutil builds with
GCC 4.8.2) orginates from the call to

  InvalidateRange( SwCharRange( GetOfst(), COMPLETE_STRING ) );

in SwTxtFrm::Prepare further down in txtfrm.cxx, which inlines the IsIdxInside
code to

  if( GetOfst() > GetOfst() + COMPLETE_STRING )

where COMPLETE_STRING is SAL_MAX_INT32.  The problem is that before
ba27366f3d6bc6b209ecd5c5cb79a9ee5315316a "Resolves: #i17171# Writer paragraph
cannot be longer than 65534 characters" that code would inline to

  if( GetOfst() > GetOfst() + STRING_LEN )

where STRING_LEN was 0xFFFF, so the calculation on effectively 16-bit quantities
stayed nicely in the bounds of (32-bit) int.

Change-Id: I958514e52e9102236844eefa4fe92a401be6ab01