rigol-ds: silence potential NULL dereference compiler warning

Check pointers' validity before dereferencing them.

This was reported by clang's scan-build.

src/hardware/rigol-ds/api.c

@@ -752,6 +752,8 @@ static int config_list(uint32_t key, GVariant **data,
 	case SR_CONF_DEVICE_OPTIONS:
 		if (!cg)
 			return STD_CONFIG_LIST(key, data, sdi, cg, scanopts, drvopts, devopts);
+		if (!devc)
+			return SR_ERR_ARG;
 		if (cg == devc->digital_group) {
 			*data = std_gvar_array_u32(NULL, 0);
 			return SR_OK;