virstring: Resolve Coverity FORWARD_NULL
virstring: Resolve Coverity FORWARD_NULL

Perhaps a false positive, but since Coverity doesn't understand the
relationship between the 'count' and the 'strings', rather than leave
the chance the on input 'strings' is NULL and causes a deref - just
check for it and return

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

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