Fix segfault on malformed message-part paths

Fix segfault on malformed message-part paths

When the part path is multilevel and a non-terminal index is out-of-range, the code would try obtain a QAbstractItemModel * from an empty QModelIndex, which is a nullptr, and then try to call something through that model. That's a segfault.

call pathToPart() on Trojita-specific URLs only

The code was feeding all URLs through the MsgPartNetAccessManager::pathToPart(). This had a side effect that those URLs whose paths looked like a path from a valid Trojita-specific URL would try to walk the part tree. This in itself is not a problem (pathToPart() is public API and therefore it should not make any assumptions about URL validity), but as the previous commit shows, sometimes mistakes happen. The funny URL looks like this one:

http://example.org/2022/06/11/foo.png

Since pathToPart() only makes sense on Trojita-specific URLs, call that just in that case.

refactor: pathToPart: simplify

The "not a number" case is not special in any way, so let's reflect that in the code.

refactor: pathToPart: there's no special handling for empty paths

The comments would try to imply that there's some magic for a path that's just /. There's no such check, so let's remove that comment.

Also, don't talk about QString::SkipEmptyParts which the code used to use many, many years ago.

Also move the ok to where it belongs.

Fixes: 35ff866e Use QByteArray for various IMAP-related paths