Fix confusion about number of subplans in partitioned INSERT setup.

Fix confusion about number of subplans in partitioned INSERT setup.

ExecInitModifyTable() thought there was a plan per partition, but no,
there's only one.  The problem had escaped detection so far because there
would only be visible misbehavior if there were a SubPlan (not an InitPlan)
in the quals being duplicated for each partition.  However, valgrind
detected a bogus memory access in test cases added by commit 4f7a95be2,
and investigation of that led to discovery of the bug.  The additional
test case added here crashes without the patch.

Patch by Amit Langote, test case by me.

Discussion: https://postgr.es/m/10974.1497227727@sss.pgh.pa.us