Fix a couple of planner bugs in Gather Merge.

Robert Haas <rhaas@postgresql.org>

Commit: 0ee92e1c9b570f90d043e5b033a19b67497ea1f6
Author: Robert Haas <rhaas@postgresql.org>
Date: 2017-03-09T17:06:49Z
Releases: 10.0
Fix a couple of planner bugs in Gather Merge.

Neha Sharma reported these to Rushabh Lathia just after I commit
355d3993c53ed62c5b53d020648e4fbcfbf5f155 went in.  The patch is
Rushabh's, with input from me.

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