opam-admin make: first check that there is a repo
Closes #702
Fix regression introduced in 2641bf825908cf4d53e10453c9355c81235c948b

(thx @AltGr for noticing)
Merge branch 'master' of github.com:OCamlPro/opam
Merge pull request #739 from AltGr/mk-repo-check

opam-admin make: first check that there is a repo