storage: Resolve Coverity FORWARD_NULL
storage: Resolve Coverity FORWARD_NULL

Coverity points out it's possible for one of the virCommand{Output|Error}*
API's to have not allocated 'output' and/or 'error' in which case the
strstr comparison will cause a NULL deref

Signed-off-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>

jardasgit/libvirt,datto/libvirt,agx/libvirt,datto/libvirt,VenkatDatta/libvirt,crobinso/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jardasgit/libvirt,olafhering/libvirt,agx/libvirt,olafhering/libvirt,datto/libvirt,nertpinx/libvirt,taget/libvirt,crobinso/libvirt,nertpinx/libvirt,andreabolognani/libvirt,libvirt/libvirt,crobinso/libvirt,agx/libvirt,nertpinx/libvirt,libvirt/libvirt,jfehlig/libvirt,taget/libvirt,andreabolognani/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,olafhering/libvirt,taget/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,olafhering/libvirt,eskultety/libvirt,rlaager/libvirt,libvirt/libvirt,zippy2/libvirt,datto/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,datto/libvirt,zippy2/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,agx/libvirt,eskultety/libvirt,jfehlig/libvirt,jardasgit/libvirt,rlaager/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,zippy2/libvirt,VenkatDatta/libvirt,rlaager/libvirt,agx/libvirt,rlaager/libvirt,zippy2/libvirt,eskultety/libvirt,andreabolognani/libvirt,eskultety/libvirt,nertpinx/libvirt,rlaager/libvirt,taget/libvirt,libvirt/libvirt,crobinso/libvirt,taget/libvirt