Fix dialyzer error in kv_mrc_map

This was uncovered once basho/riak_pipe
a33ac811ddefff0f0a9c04cbec4611a1537ed98a was committed.
Merge pull request #941 from basho/bugfix/pipe-worker-init-spec

Fix dialyzer error in kv_mrc_map

Reviewed-by: seancribbs