Detect incorrect paramenter in findProtocol

Summary:
Add two Q_ASSERT to detect incorrect protocols like:

• "" (empty parameter)
• smb:/// (the right one is smb)
• zip:/home/user/Download/......./Encoder.php (the right one is zip)

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11013