SystemUI: prevent NPE on clear recents double press

Pressing Clear all button twice fast or while the list
is clearing but not yet fully cleared causes sysui to crash due to
mRecentTaskDescriptions being null: (http://pastebin.com/2gRFvf8k)

Change-Id: I23d10edfd336041739fe80352ee01987e671dbd7

Conflicts:
	packages/SystemUI/src/com/android/systemui/recent/RecentsPanelView.java