child-cfg: Allow passing NULL as proposal to add_proposal()

Making the API consistent to the one of ike_cfg.

src/libcharon/config/child_cfg.c

@@ -149,7 +149,10 @@ METHOD(child_cfg_t, get_name, char*,
 METHOD(child_cfg_t, add_proposal, void,
 	private_child_cfg_t *this, proposal_t *proposal)
 {
-	this->proposals->insert_last(this->proposals, proposal);
+	if (proposal)
+	{
+		this->proposals->insert_last(this->proposals, proposal);
+	}
 }
 
 METHOD(child_cfg_t, get_proposals, linked_list_t*,

src/libcharon/config/child_cfg.h

@@ -73,10 +73,10 @@ struct child_cfg_t {
 	 * Add a proposal to the list.
 	 *
 	 * The proposals are stored by priority, first added
-	 * is the most preferred.
-	 * After add, proposal is owned by child_cfg.
+	 * is the most preferred. It is safe to add NULL as proposal, which has no
+	 * effect. After add, proposal is owned by child_cfg.
 	 *
-	 * @param proposal		proposal to add
+	 * @param proposal		proposal to add, or NULL
 	 */
 	void (*add_proposal) (child_cfg_t *this, proposal_t *proposal);