libs: decoder: h265: check for null

libs: decoder: h265: check for null

Coverity scan bug:

Dereference after null check: Either the check against null is
unnecessary, or there may be a null pointer dereference.

While looking for hte lowest poc, according to rest of the code, the
picture in the dbp (decoded picture buffer) might be NULL, thus we
could check for a NULL picture before assigned as found.

Also, split a comma operator because it is considered as a bad
practice because it possible side effects.