Fix bug in Map::TraverseTransitionTree.

If map has an empty DescriptorArray we have to set map_or_index_field to NULL
otherwise we will
reset iteration state for a previously visited map in the transition tree which
has a non-empty
DescriptorArray. This might result in visiting the same map several times.

R=whesse@chromium.org
BUG=v8:1526
Review URL: http://codereview.chromium.org/7333008

git-svn-id: http://v8.googlecode.com/svn/branches/3.3@8604 ce2b1a6d-e550-0410-aec6-3dcde31c8c00