ART: Fix IsLinearOrderWellFormed in liveness analysis

Graph may contain nullptr in list of basic blocks when
the basic block is removed. So we must handle nullptr explicitly.

Bug: 28252537
Change-Id: If430939b8fe793e4a9c974936180d06c1a9f90c6
Signed-off-by: Serguei Katkov <serguei.i.katkov@intel.com>