ART: Implicit null check should break def tracking

Implicit null check can provoke exception that needs to be sure all VRs are saved on stack.
The fix is to reset the def tracking system at the moment of adding an implicit null check.

  b/18368901

Signed-off-by: Pavel Vyssotski <pavel.n.vyssotski@intel.com>

(cherry picked from commit 9c3617a8)

Change-Id: If7d705b8417cf29076da9ce19ec54d698f3cb0d5