charon-nm: No additional secrets are required once a password has been entered

Recent versions of NM will call need_secrets() as long as it returns TRUE,
but then fail as the number of calls is limited by an assert.

Fixes #547.

src/charon-nm/nm/nm_service.c

@@ -660,6 +660,10 @@ static gboolean need_secrets(NMVPNPlugin *plugin, NMConnection *connection,
 					key->destroy(key);
 					return FALSE;
 				}
+				else if (nm_setting_vpn_get_secret(settings, "password"))
+				{
+					return FALSE;
+				}
 			}
 		}
 		else if (streq(method, "smartcard"))