nat: Provide information of when a msg does not contain a CI

When we fail to find a CI in a message that should contain it
then print the message so we can analyze the issue and improve
the code later on.
diff --git a/openbsc/src/nat/bsc_mgcp_utils.c b/openbsc/src/nat/bsc_mgcp_utils.c
index f038342..05963d3 100644
--- a/openbsc/src/nat/bsc_mgcp_utils.c
+++ b/openbsc/src/nat/bsc_mgcp_utils.c
@@ -348,11 +348,16 @@
 {
 	int ci;
 	char *res = strstr(str, "I: ");
-	if (!res)
+	if (!res) {
+		LOGP(DMGCP, LOGL_ERROR, "No CI in msg '%s'\n", str);
 		return CI_UNUSED;
+	}
 
-	if (sscanf(res, "I: %d", &ci) != 1)
+	if (sscanf(res, "I: %d", &ci) != 1) {
+		LOGP(DMGCP, LOGL_ERROR, "Failed to parse CI in msg '%s'\n", str);
 		return CI_UNUSED;
+	}
+
 	return ci;
 }