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>
diff --git a/compiler/optimizing/ssa_liveness_analysis.h b/compiler/optimizing/ssa_liveness_analysis.h
index 719feec..40dab74 100644
--- a/compiler/optimizing/ssa_liveness_analysis.h
+++ b/compiler/optimizing/ssa_liveness_analysis.h
@@ -971,7 +971,7 @@
 
   bool IsLinearOrderWellFormed(const HGraph& graph) {
     for (HBasicBlock* header : graph.GetBlocks()) {
-      if (!header->IsLoopHeader()) {
+      if (header == nullptr || !header->IsLoopHeader()) {
         continue;
       }