pdr: pldm_entity_association_pdr_add(): Exit early on bad arguments

It's not possible to perform the conversion if one or both of tree or
repo are NULL. Explicitly document this case.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I3c77b0d99c5c0badc457a4117312105f22bf5a61
diff --git a/src/pdr.c b/src/pdr.c
index bacdd5d..3223cfb 100644
--- a/src/pdr.c
+++ b/src/pdr.c
@@ -918,6 +918,9 @@
 {
 	assert(tree != NULL);
 	assert(repo != NULL);
+	if (!tree || !repo) {
+		return;
+	}
 
 	entity_association_pdr_add(tree->root, repo, NULL, 0, is_remote,
 				   terminus_handle, 0);