Use unique_ptr to destroy per-bfd object

In some cases, the objfile owns the per-bfd object.  This is yet
another object that can sometimes be destroyed before the registry is
destroyed, possibly reslting in a use-after-free.  Also, I noticed
that the condition for deleting the object is not the same as the
condition used to create it -- so it could possibly result in a memory
leak in some situations.  This patch fixes the problem by introducing
a new unique_ptr that holds this object when necessary.