ContextView: fix crash in addCollapseAnimation, dedicated to Mamarok

With some gdb fu, I was able to investigate that the root cause of the
bug was invalid static_cast<>, I wish static_casts were (almost) never
used in Amarok.

This crash-fix is a day-late present for Myriam Schweingruber's
birthday - cheers, Myriam! :-)

[this crash existed (in its current form) since at least 2011-07-14 as
reported by Marian Kyral. We thought we have fixed it already, but we
have in fact just reduced its probability by defaulting to a different
configuration wrt collapsing applets. My last patch touching this
7ff8068a was rather a blind guess and debugging patch than
a real fix.]

BUG: 258741
FIXED-IN: 2.6
CCMAIL: Myriam Schweingruber <myriam@kde.org>
DIGEST: Amarok crash-fix, please mention dedication ;-)