CallIC customization stubs must accept that a vector slot is cleared.

The CallIC Array custom IC stub read from the type vector, expecting
to get an AllocationSite. But there are paths in the system where a
type vector can be re-created with default values, even though we
currently grant an exception to clearing of vector slots with
AllocationSites in them at gc time.

BUG=392114
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/418023002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22668 ce2b1a6d-e550-0410-aec6-3dcde31c8c00