Provided better sanity-checking in _is_oid_prefix, against null results from Redis
Provided better sanity-checking in _is_oid_prefix, against null results from Redis

The problem wasn't noticed on EL7, as apparently the Perl on EL7 has different
behavior when doing the following:

* calling `substr` with bounds that are outside the relevant string
* calling `split` on an undef value

Said different behavior led to things "just working" on EL7, but not on EL6.

GlobalNOC/simp,GlobalNOC/simp,GlobalNOC/simp,GlobalNOC/simp,GlobalNOC/simp