odk/unoapploader/win: fix cppcheck error

 Common realloc mistake: 'buf' nulled but not freed upon failure

Change-Id: I8a793e0267c1bb7086bb4f13aabb374faa94f7a4
Reviewed-on: https://gerrit.libreoffice.org/10702
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>