ike-cfg: Allow passing NULL to add_proposal()

This simplifies adding default proposals with constructors potentially
returning NULL.

src/libcharon/config/ike_cfg.c

@@ -281,7 +281,10 @@ METHOD(ike_cfg_t, get_dscp, u_int8_t,
 METHOD(ike_cfg_t, add_proposal, void,
 	private_ike_cfg_t *this, proposal_t *proposal)
 {
-	this->proposals->insert_last(this->proposals, proposal);
+	if (proposal)
+	{
+		this->proposals->insert_last(this->proposals, proposal);
+	}
 }
 
 METHOD(ike_cfg_t, get_proposals, linked_list_t*,

src/libcharon/config/ike_cfg.h

@@ -148,9 +148,10 @@ struct ike_cfg_t {
 	 * Adds a proposal to the list.
 	 *
 	 * The first added proposal has the highest priority, the last
-	 * added the lowest.
+	 * added the lowest. It is safe to add NULL as proposal, which has no
+	 * effect.
 	 *
-	 * @param proposal		proposal to add
+	 * @param proposal		proposal to add, or NULL
 	 */
 	void (*add_proposal) (ike_cfg_t *this, proposal_t *proposal);