ART: Allow PackedSwitch instructions with zero targets

Optimizing and the interpreter wrongly assumed that a PackedSwitch
always has at least one target. This patch removes the corresponding
DCHECKs and adds a regression test case.

This is a resubmission of CL I32b7033ed38de6f1d1a6ee5d5bf12f3a47c9b37e

Bug: 21863783
Change-Id: I04e6e124bdd16591ba27c79490e6ce183c36b691
(cherry picked from commit 2ef645ba)