json input link with calc->expr->pva not working

The database below fails with
```
pvaLink void {anonymous}::pvaOpenLink(DBLINK*) fails MTEST-JSON:inp_json_calc: DBLINK* corrupt
```

system details:
linux x86_64
EPICS R7.0.7-DEV, as well as R7.0.7

database
```
record(ai, "MTEST-JSON:A1") {
    field(VAL, 1)
    field(PINI, 1)
}

record(ai, "MTEST-JSON:inp_json_calc") {
    field(INP,
        {
            "calc": {
                "expr": "A|B|C",
                "args": [
                    {
                        "const": 0
                    },
                    0,
                    {
                        "pva": {
                            "pv": "MTEST-JSON:A1",
                            "proc": "CP"
                        }
                    }
                ]
            }
        }
    )
}
```