Fix uninitialized memory reference.

Without this, when partdesc->nparts == 0, we end up calling
ExecBuildSlotPartitionKeyDescription without initializing values
and isnull.

Reported by Coverity via Michael Paquier.  Patch by Michael Paquier,
reviewed and revised by Amit Langote.

Discussion: http://postgr.es/m/CAB7nPqQ3mwkdMoPY-ocgTpPnjd8TKOadMxdTtMLvEzF8480Zfg@mail.gmail.com