Ensure that BitTableAccessor refers to non-null table.

Hopefully this should make clang analyzer happy.

Test: test-art-host-gtest-stack_map_test
Change-Id: I19c9048302dd47dd262842e0b58ed139f83fa1f6
diff --git a/runtime/stack_map.cc b/runtime/stack_map.cc
index f40168b..5472d4c 100644
--- a/runtime/stack_map.cc
+++ b/runtime/stack_map.cc
@@ -44,7 +44,7 @@
       return *it;
     }
   }
-  return StackMap();
+  return stack_maps_.GetInvalidRow();
 }
 
 // Scan backward to determine dex register locations at given stack map.