[gck] Fix crash in enumerator when C_FindObjects fails.
[gck] Fix crash in enumerator when C_FindObjects fails.

If C_FindObjects failed when used in a gck_enumerate_xxx()
and friends, then we would crash.

stefwalter/gcr,Distrotech/gcr,Distrotech/gcr,stefwalter/gcr,stefwalter/gcr,Distrotech/gcr,Distrotech/gcr,stefwalter/gcr