Avoid segv when the application doesn't set up the authentication callback.
Avoid segv when the application doesn't set up the authentication
callback.

* auth/auth_digest.c:
  (serf__handle_digest_auth): Fail authn early when credentials 
   callback is not set.

muthusuba/serf,Manouchehri/serf,idomon/serf,liuzongquan/serf,krime/serf,wzugang/serf,liuzongquan/serf,justinweb/serf,sworda/serf,blueszhangsh/serf,liuzongquan/serf,TommyU/serf,wzugang/serf,sworda/serf,TommyU/serf,justinweb/serf,idomon/serf,idomon/serf,Manouchehri/serf,Manouchehri/serf,wzugang/serf,TommyU/serf,muthusuba/serf,Manouchehri/serf,wzugang/serf,liuzongquan/serf,blueszhangsh/serf,blueszhangsh/serf,justinweb/serf,muthusuba/serf,justinweb/serf,TommyU/serf,krime/serf,sworda/serf,idomon/serf,krime/serf,muthusuba/serf,blueszhangsh/serf,krime/serf,sworda/serf