evas_smart: Add a check for zero size before malloc

evas_smart: Add a check for zero size before malloc

Summary:
	Size may be zero while allocating interfaces private data,
	when NULL is passed in EVAS_SMART_SUBCLASS_IFACE_NEW for ifaces argument.

	If size is 0, then malloc() returns either NULL, or a unique pointer
	value that can later be successfully passed to free(). It is implementation
	dependent. This case would be likely to occur in case of smart classes with
	no interfaces.

@fix

Signed-off-by: Vaibhav Gupta <g.vaibhav1@samsung.com>
Reviewers: raster, Hermet, tasn, cedric

Subscribers: SanghyeonLee, sachin.dev, singh.amitesh, cedric

Differential Revision: https://phab.enlightenment.org/D2679