Fix crash in wxExecute() introduced by r73406.

Don't dereference potentially NULL wxProcess pointer.

See #14931.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73425 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix a crash in wxExecute() in wxMSW too.

Don't dereference potentially NULL wxProcess pointer unconditionally.

This should have been together with the changes of r73425, see #14931.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73426 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775