dm: core: fix no null pointer detection in ofnode_get_addr_size_index()

Fixed a defect of a null pointer being discovered by Coverity Scan:
   CID 331544:  Null pointer dereferences  (REVERSE_INULL)
   Null-checking "size" suggests that it may be null, but it has already been
   dereferenced on all paths leading to the check.

Signed-off-by: Chen Guanqiao <chenguanqiao@kuaishou.com>