[ Runtime | Extension ] json_decode parses invalid JSON strings and returns a result for them

**Describe the bug**
When given an invalid JSON string which is defective in one of the manners described below, `json_decode()` returns a result. This JSON is not strictly valid according to the spec and should ideally result in a parse error.

**Standalone code, or other way to reproduce the problem**
```HACK
function json_decode_adapted(string $json, bool $assoc): (string, ?string) {
$error = null;
$return = json_decode_with_error(
$json,
inout $error,
$assoc,
512,
\JSON_FB_HACK_ARRAYS,
);
$err_str = $error is null ? null : $error[1];
return tuple(json_encode($return), $err_str);
}

function json_decode_both(string $name, string $json): void {
list($std_res, $std_err) = json_decode_adapted($json, false);
list($assoc_res, $assoc_err) = json_decode_adapted($json, true);
printf("======= %s =======\n", $name);

if ($std_err is string && $assoc_err is string) {
printf(
"Both stdClass and assoc mode rejected this, stdClass: %s, assoc: %s\n\n",
$std_err,
$assoc_err,
);
return;
}

if ($std_err !== $assoc_err) {
printf(
"stdClass mode and assoc mode disagree about the error\nstdClass: %s\nassoc: %s\n\n",
$std_err ?? '',
$assoc_err ?? '',
);
}

if ($std_res !== $assoc_res) {
printf(
"The results differ\nstdClass: %s\nassoc: %s\n\n",
$std_res,
$assoc_res,
);
} else {
printf("The result %s\n\n", $assoc_res);
}
}

const dict EXAMPLES = dict[
'single minus token' => '-',
'minus token in array' => '[-]',
'invalid numeric literal' => '01',
'negative invalid numeric literal' => '-01',
'invalid numeric literal in array' => '[01]',
'negative invalid numeric literal in array' => '[-01]',
'form feed as whitespace in array' => "[1\f,2]",
'form feed as whitespace in object' => <<>
function main(): void {
foreach (EXAMPLES as $name => $json) {
json_decode_both($name, $json);
}
}

```

Steps to reproduce the behavior:
1. Run the script above
2. The correct result would be `Both stdClass and assoc mode rejected this...` for every decode attempt
3. Observe the results (see Actual behavior)

**Expected behavior**

All these json snippets are syntactically invalid, but not all of them are rejected. These examples should all be a `Syntax error` or a `Control character error`.

**Actual behavior**
```
======= single minus token =======
stdClass mode and assoc mode disagree about the error
stdClass: Syntax error
assoc:

The results differ
stdClass: null
assoc: 0

======= minus token in array =======
stdClass mode and assoc mode disagree about the error
stdClass: Syntax error
assoc:

The results differ
stdClass: null
assoc: [0]

======= invalid numeric literal =======
The result 1

======= negative invalid numeric literal =======
The result -1

======= invalid numeric literal in array =======
Both stdClass and assoc mode rejected this, stdClass: Syntax error, assoc: Syntax error

======= negative invalid numeric literal in array =======
stdClass mode and assoc mode disagree about the error
stdClass: Syntax error
assoc:

The results differ
stdClass: null
assoc: [-1]

======= form feed as whitespace in array =======
stdClass mode and assoc mode disagree about the error
stdClass: Control character error, possibly incorrectly encoded
assoc:

The results differ
stdClass: null
assoc: [1,2]

======= form feed as whitespace in object =======
stdClass mode and assoc mode disagree about the error
stdClass: Control character error, possibly incorrectly encoded
assoc:

The results differ
stdClass: null
assoc: {"bad":"result"}

```

**Environment**
- Operating system
> Ubuntu 20.04
- Installation method
> apt-get with dl.hhvm.com repository (also tested in Docker)
- HHVM Version
```
HipHop VM 4.88.0 (rel)
Compiler: 1608056886_380600296
Repo schema: d1ae8e21bf3419a65f12a010527485564e719d07
hackc-9e3bb09ecef0a93ae0917fb44ed4e2434e990c17-4.88.0
```

**Additional context**
The list of examples is not meant to be exhaustive.