BUG #16171: Potential malformed JSON in explain output

The following bug has been logged on the website:

Bug reference: 16171
Logged by: Mahadevan Ramachandran
Email address: mahadevan(at)rapidloop(dot)com
PostgreSQL version: 12.1
Operating system: any
Description:

Refer src/backend/commands/explain.c, version 12.1.

When a plan node has children, the function ExplainNode starts a JSON array
with the key "Plans" (line 1955), like so:

"Plans": [

with the intention of creating an array of "Plan" objects, one for each
child:

"Plans": [
{ .. a child plan goes here ..},
{ .. a child plan goes here ..}
]

However, if the node (the current, parent one) is of a certain type (see
switch at line 1975), then ExplainMemberNodes is called, which does this
(lines 3335-6):

if (nsubnodes < nplans)
ExplainPropertyInteger("Subplans Removed", NULL, nplans - nsubnodes,
es);

This can potentially cause a malformed JSON output like this:

"Plans": [
{ .. a child plan goes here ..},
"Subplans Removed": 5,
{ .. a child plan goes here ..}
]

I don't have a sample explain output that exhibits this error, this was
found while reviewing the code.