Sanity check item for being non-null.

This normally shouldn't happen, but experience has shown that there are
some rare corner cases where it indeed happens. The exact conditions are
unclear, but it's likely related to an ACTION_CANCEL event being sent
out before the ACTION_UP event.
In order to not crash in weird, hard-to-debug corner cases, just check
whether we actually have an item to process.

This fixes http://code.google.com/p/cyanogenmod/issues/detail?id=4985.