oap_client: make use of OAP optional: disable for NULL config

When passing a NULL config to osmo_oap_client_init(), set OAP to disabled
state. Along with the previous fix that ensures message rejection in the
disabled state, this makes use of OAP in the GSUP client optional.

oap_client_test: expect null config to set state to disabled.

Related: OS#1592
Change-Id: Ie4d622fcfd24cb7d89d19f93e4b2571d8fadd1a3
diff --git a/openbsc/src/libcommon/oap_client.c b/openbsc/src/libcommon/oap_client.c
index e372ede..54b71e4 100644
--- a/openbsc/src/libcommon/oap_client.c
+++ b/openbsc/src/libcommon/oap_client.c
@@ -35,6 +35,9 @@
 {
 	OSMO_ASSERT(state->state == OAP_UNINITIALIZED);
 
+	if (!config)
+		goto disable;
+
 	if (config->client_id == 0)
 		goto disable;