pgsql: Add stack depth checks to key recursive functions in backend/nod

Add stack depth checks to key recursive functions in backend/nodes/*.c.

Although copyfuncs.c has a check_stack_depth call in its recursion,
equalfuncs.c, outfuncs.c, and readfuncs.c lacked one. This seems
unwise.

Likewise fix planstate_tree_walker(), in branches where that exists.

Discussion: /messages/by-id/30253.1544286631@sss.pgh.pa.us

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/552c310ba4ec072a2df48a08722f0af2e3703fdf

Modified Files
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src/backend/nodes/equalfuncs.c | 4 ++++
src/backend/nodes/outfuncs.c | 4 ++++
src/backend/nodes/readfuncs.c | 4 ++++
3 files changed, 12 insertions(+)