ctl: use condition statements instead of assert() for new APIs to add an element set

ctl: use condition statements instead of assert() for new APIs to add an element set

Usage of assert() is not better practice of programming as shared library
APIs. They should return appropriate error code to promote applications to
handle error state.

This commit applies condition statements with return value of -EINVAL,
instead of assert(). As a backward compatibility for existent applications,
old APIs still call assert().

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>