Fix exit on last TLW logic after the change of r75630.

The changes in this revision meant to prevent the closure of transient dialogs
from quitting the application (see #15880) prevented any application using AUI,
including the aui sample, from exiting as the AUI utility frames deleted
during the main frame destruction were returning false from their
IsLastBeforeExit() now.

Fix this by relaxing the check and ignoring the parent if it is already being
deleted anyhow -- in this case there is no danger of closing it accidentally.

Closes #15894.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75832 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Avoid crashes when deleting owned top level windows.

Don't delay the TLW destruction if it has a parent and its parent is already
being deleted: we can't delay the inevitable in this case and only succeed in
making the program crash if we try.

Closes #15743.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75833 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775