stge: fix null pointer dereference
stge_attach() could fail at line 464, sc->sc_spec remains NULL when
calling stge_detach(), thus bus_release_resources() at line 704 will
trigger null pointer dereference. We need to check the nulliness before
calling bus_release_resources().
PR: 258420
Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34629