pdr: pldm_entity_association_tree_destroy(): Exit early on bad argument

There's no need to rely on the violence of assert() in this instance.
Just bail out if tree is NULL.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: If3d1d92015f37d56ab389bed7f20892db9b63e13
diff --git a/src/pdr.c b/src/pdr.c
index 285aae6..3c9db5d 100644
--- a/src/pdr.c
+++ b/src/pdr.c
@@ -699,7 +699,9 @@
 LIBPLDM_ABI_STABLE
 void pldm_entity_association_tree_destroy(pldm_entity_association_tree *tree)
 {
-	assert(tree != NULL);
+	if (!tree) {
+		return;
+	}
 
 	entity_association_tree_destroy(tree->root);
 	free(tree);