Possible issue with unserializing Cookie data

[trepmal]

I discovered this while working on #13. I believe I confirmed it's not a regression, but would welcome confirmation.

Issue

When re-flagging a comment, I do not get the expected "already flagged" message.

Reproduce

1. Activate the plugin on a site running any default theme but Twenty Twenty (See if there are better options than comment_reply_link #14).
2. Add an approve at least one comment for testing on.
3. View the comment on the front-end and click "Report Comment"
4. Note you'll get a sfrc_flags cookie value something like this eyIxOTAzMCI6MX0%3D

The problem is during unserialization

safe-report-comments/safe-report-comments.php

Lines 248 to 251 in d28439f

	private function unserialize_cookie( $value ) {
	$data = json_decode( base64_decode( $value ) );
	return $this->clean_cookie_data( $data );
	}

Problem one is the base64 decoding leaves a trailing character

wp> $base64 = base64_decode( 'eyIxOTAzMCI6MX0%3D' )
=> string(12) "{"19030":1}7"

Which then prevents json decoding

wp> json_decode( $base64 );
=> NULL

Because the data ends up as null (which is "cleaned" into an empty array), our comment isn't found in the data so is not considered "already flagged".

Note: It may be difficult to fully see this on the front-end due to the transients fallback. Conditional blocks at

safe-report-comments/safe-report-comments.php

Line 298 in d28439f

if ( $transient = get_transient( md5( $this->_storagecookie . $remote_addr ) ) ) {

and

safe-report-comments/safe-report-comments.php

Line 346 in d28439f

if ( !$transient ) {

can be commented out for testing.