Fix error-cleanup mistakes in exec_stmt_call().

Commit 15c729347 was a couple bricks shy of a load: we need to
ensure that expr->plan gets reset to NULL on any error exit,
if it's not supposed to be saved.  Also ensure that the
stmt->target calculation gets redone if needed.

The easy way to exhibit a problem is to set up code that
violates the writable-argument restriction and then execute
it twice.  But error exits out of, eg, setup_param_list()
could also break it.  Make the existing PG_TRY block cover
all of that code to be sure.

Per report from Pavel Stehule.

Discussion: https://postgr.es/m/CAFj8pRAeXNTO43W2Y0Cn0YOVFPv1WpYyOqQrrzUiN6s=dn7gCg@mail.gmail.com

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