pgsql: Ensure dummy paths have correct required_outer if rel isparamet

Ensure dummy paths have correct required_outer if rel is parameterized.

The assertions added by commits 34ea1ab7f et al found another problem:
set_dummy_rel_pathlist and mark_dummy_rel were failing to label
the dummy paths they create with the correct outer_relids, in case
the relation is necessarily parameterized due to having lateral
references in its tlist.  It's likely that this has no user-visible
consequences in production builds, at the moment; but still an assertion
failure is a bad thing, so back-patch the fix.

Per bug #15694 from Roman Zharkov (via Alexander Lakhin)
and an independent report by Tushar Ahuja.

Discussion: https://postgr.es/m/15694-74f2ca97e7044f7f@postgresql.org
Discussion: https://postgr.es/m/7d72ab20-c725-3ce2-f99d-4e64dd8a0de6@enterprisedb.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0255329a780f9c52de5eb5fb73205e12c93deede

Modified Files
--------------
src/backend/optimizer/path/allpaths.c |  4 +++-
src/backend/optimizer/path/joinrels.c |  4 +++-
src/test/regress/expected/join.out    | 29 +++++++++++++++++++++++++++++
src/test/regress/sql/join.sql         |  8 ++++++++
4 files changed, 43 insertions(+), 2 deletions(-)