pdr: Add preconditions for pldm_entity_association_tree_copy_root()

pldm_entity_association_tree_copy_root() requires that the arguments
passed point to valid objects. Ensure this is documented, and add
assert()s for early warning of precondition violation in debug builds

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I15f4f10d5e7a8cebfffae94bb868a905c7c98f3d
diff --git a/src/pdr.c b/src/pdr.c
index 8865002..ab35d2a 100644
--- a/src/pdr.c
+++ b/src/pdr.c
@@ -1216,6 +1216,9 @@
 	pldm_entity_association_tree *org_tree,
 	pldm_entity_association_tree *new_tree)
 {
+	assert(org_tree != NULL);
+	assert(new_tree != NULL);
+
 	new_tree->last_used_container_id = org_tree->last_used_container_id;
 	entity_association_tree_copy(org_tree->root, &(new_tree->root));
 }