refs.c: ensure struct whose member may be passed to realloc is initialized

refs.c: ensure struct whose member may be passed to realloc is initialized

The variable "refs" is allocated on the stack but is not initialized.  It
is passed to read_packed_refs(), and its struct members may eventually be
passed to add_ref() and ALLOC_GROW().  Since the structure has not been
initialized, its members may contain random non-zero values.  So let's
initialize it.

The call sequence looks something like this:

   resolve_gitlink_packed_ref(...) {

       struct cached_refs refs;
       ...
       read_packed_refs(f, &refs);
       ...
   }

   read_packed_refs(FILE*, struct cached_refs *cached_refs) {
       ...
       add_ref(name, sha1, flag, &cached_refs->packed, &last);
       ...
   }

   add_ref(..., struct ref_array *refs, struct ref_entry **) {
       ...
       ALLOC_GROW(refs->refs, refs->nr + 1, refs->alloc);
   }

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>