Fix crash when first loading AlphabetIndexer sample

On Google Edition HTC One, the AlphabetIndexer sample crashes the first time it is opened:

        E  android.database.CursorIndexOutOfBoundsException: Index 0 requested, with a size of 0
                        E  	at android.database.AbstractCursor.checkPosition(AbstractCursor.java:430)
                        E  	at android.database.AbstractWindowedCursor.checkPosition(AbstractWindowedCursor.java:136)
                        E  	at android.database.AbstractWindowedCursor.getString(AbstractWindowedCursor.java:50)
                        E  	at android.database.CursorWrapper.getString(CursorWrapper.java:118)
                        E  	at android.widget.AlphabetIndexer.getSectionForPosition(AlphabetIndexer.java:258)
                        E  	at com.manuelpeinado.multichoiceadapter.demo.alphabetindexersample.AlphabetIndexerCursorAdapter.getSectionForPosition(AlphabetIndexerCursorAdapter.j
                           ava:93)
                        E  	at android.widget.FastScroller.getThumbPositionForListPosition(FastScroller.java:647)
                        E  	at android.widget.FastScroller.onScroll(FastScroller.java:457)
                        E  	at android.widget.AbsListView.invokeOnItemScrollListener(AbsListView.java:1452)
                        [.....]

Not sure this is the best way to fix it but it did stop the crash.