[ENHANCEMENT] Make jumpurl for dmailerping less error-prone

[BastianBalthasarBux]

This is really only a nice-to-have:

If the dmailerping.gif (statistics) is included with the domain prefixed, e.g.:
<img src="https://example.example/typo3conf/ext/direct_mail/Resources/Public/Icons/dmailerping.gif" width="1" height="1" dmailerping="1" do_not_embed="1" alt="" border="0" />

You will notice an error in TYPO3 backend (Logging module), because Hook/JumpurlController will throw an exception.

What happens?
The above results in an URL in the mailing like this:
https://example.example/newsletter-test/?mid=6&aC=d0eff497&jumpurl=https://example.example/typo3conf/ext/direct_mail/Resources/Public/Icons/dmailerping.gif

$checkPath then, around line 154, gets something like that:
/path/to/vhosts/example.example/htdocs/https://example.example/typo3conf/ext/direct_mail/Resources/Public/Icons/dmailerping.gif

And this is, obviously, not a valid path, and thus "not allowed" and results in forementioned exception.

Maybe, instead of fixing that code here, it would be better to check for the complete URI when generating the mail content and correcting the URL already at that point.