kcms/plasmasearch: don't crash on dragging favorties to the end

snap it back to the end instead.

this happens because the pluginselector doodad is a singular list view
with sections. when the user drags an item from the favorites section to
the available section, to reorder an item at the bottom, the new drop
index is inside the model but not inside the m_favoriteMetaDataList.
i.e. its index is model-bound not m_favoriteMetaDataList-bound. we
therefore need to clamp the index at the upper bounds of
m_favoriteMetaDataList manually so as to not work with invalid indexes

BUG: 481594
(cherry picked from commit 79070f06)