pdr: pldm_entity_association_tree_destroy_root(): Exit early on bad arg

Also identify the condition in the doxygen commentary in the header.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I706dc4364424163e51d6d2b17168f3dba280522b
diff --git a/src/pdr.c b/src/pdr.c
index 3ee8886..8948ea0 100644
--- a/src/pdr.c
+++ b/src/pdr.c
@@ -1231,6 +1231,10 @@
 	pldm_entity_association_tree *tree)
 {
 	assert(tree != NULL);
+	if (!tree) {
+		return;
+	}
+
 	entity_association_tree_destroy(tree->root);
 	tree->last_used_container_id = 0;
 	tree->root = NULL;