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@rapidloop.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.