Fix crash when KWallet is not available.

Summary:
Bug 398767
Currently, when the user clicks remember password when KWallet is disabled, falkon gets crashed.
This patch fixes the crash by checking if KWallet object is created and only then it adds to the folder.
The following functions are fixed:

• addEntry
• Update Entry
• updateLastUsed
• removeEntry
• removeAll

Reviewers: SGOrava, drosca

Reviewed By: SGOrava, drosca

Subscribers: drosca, falkon

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D26872