pdr: pldm_is_current_parent_child(): Return false for invalid arguments

It can't be true that a node is a child of parent if either or both of
parent or node are NULL. Explicitly document this case.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ibabaff24e12f3b53f87ecbcb127d84ab1e7773ba
diff --git a/src/pdr.c b/src/pdr.c
index 7bf0f67..5cb72f6 100644
--- a/src/pdr.c
+++ b/src/pdr.c
@@ -771,6 +771,9 @@
 {
 	assert(parent != NULL);
 	assert(node != NULL);
+	if (!parent || !node) {
+		return false;
+	}
 
 	pldm_entity_node *curr = parent->first_child;
 	while (curr != NULL) {