talloc: add pytalloc_get_name() helper

talloc: add pytalloc_get_name() helper

In several places we go

   talloc_get_name(pytalloc_get_ptr(py_obj))

which is a certain NULL derefernce if py_obj is not a talloc object.

This is a helper function that chooses to say "non-talloc object"
rather than crash.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>