srm: fix NPE when checking delegated credential

The SRM keeps "the best" delegated credential, if the user supplies one.
If the user doesn't delegate a credential then the current code throws
a NPE.  This is fixed with this patch.

Target: master
Request: 2.10
Requires-notes: no
Requires-book: no
Acked-by: Tigran Mkrtchyan
Patch: https://rb.dcache.org/r/7147/
Merge pull request #699 from paulmillar/fix/2.10/srm-delegated-credential

srm: fix NPE when checking delegated credential