Really fix stack dumps for asserts and wxStackWalker::Walk() calls.

Really fix stack dumps for asserts and wxStackWalker::Walk() calls.

The code apparently tried to compensate for the wrong "skip" values used in
the calls to wxStackWalker::Walk() by skipping too much in Walk() itself which
was wrong as it dropped the frames that should have been shown.

Fix this by skipping only the one extra (compared to Walk() itself) frame we
add in wxStackWalker Unix implementation and not 3 of them and do skip more
frames when calling Walk() from assert failure handlers.

Also fix the wrong number of frames used in ProcessFrames(): we must not
subtract the number of skipped frames, they were already skipped.

Closes #14690.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72546 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775