lib: object.h: convert precondition assertions to internal assertions

lib: object.h: convert precondition assertions to internal assertions

The `object.h` API is completely internal.

We used to have only two public reference counting modification
functions (to get and put references), but since c5b9b4417 ("lib: make
public reference count functions have strict types"), each shared object
type has its own reference counting modification functions. Therefore
it's impossible that bt_object_get_ref() or bt_object_put_ref() be
called with a unique object without at least getting a compiler warning.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I80286144f3f29af933694d5e824473abd7ec9103
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3430