gsup_server.c: properly handle negative rc from osmo_gsup_conn_ccm_get()

Related: coverity CID#210171
Change-Id: I5b56fe33cbc75679a3fffc034a53f06cd0e4043b
diff --git a/src/gsup_server.c b/src/gsup_server.c
index a5d496e..888507c 100644
--- a/src/gsup_server.c
+++ b/src/gsup_server.c
@@ -274,7 +274,7 @@
 {
 	struct osmo_gsup_conn *clnt = (struct osmo_gsup_conn *)conn->data;
 	uint8_t *addr = NULL;
-	size_t addr_len;
+	int addr_len;
 
 	LOGP(DLGSUP, LOGL_INFO, "CCM Callback\n");