Description:
------------
When parsing emails with one line the starting position and ending position of the body are the same.

if you add a second line with one character it jumps from 344 to 405, but then when you add a third line with a single character it shows end pos 407. So the issue seems to be with parsing the positions when there is just one line in body.

Test script:
---------------
<?php

$message = <<< EOF
MIME-Version: 1.0
Date: Tue, 12 Nov 2019 08:39:50 +0100
Message-ID: <CAD05h8p3WCJLqVLVLebaE03KskpD8+AGEHEjZJ1JvnJpuh2+1w@mail.gmail.com>
Subject: Text Message
From: Jamiel Sharief <somebody@gmail.com>
To: Jamiel Sharief <somebody@gmail.com>
Cc: somebody@gmail.com, Jamiel Sharief <somebody@gmail.com>
Content-Type: text/plain; charset="UTF-8"

One line email results in same starting and ending position.
EOF;

 # Create resource
 $resource = mailparse_msg_create();
 mailparse_msg_parse($resource, $message);
 $structure = mailparse_msg_get_structure($resource);

 $parts = [];
 foreach ($structure as $id) {
     $part = mailparse_msg_get_part($resource, $id);
     $parts[] = mailparse_msg_get_part_data($part);
 }

# This is a bug with starting-pos-body
print_r($parts);;

Expected result:
----------------
[starting-pos] => 0
[starting-pos-body] => 344
[ending-pos] => 404
ending-pos-body] => 404




Actual result:
--------------
[starting-pos] => 0
[starting-pos-body] => 344
[ending-pos] => 344
ending-pos-body] => 344