gsm0407_is_duplicate(): Handle error ret of gsm0407_pdisc_ctr_bin()

Change-Id: If9525694bcbc5c6c0e622e899dd634dc11ed61c4
Fixes: Coverity CID#182702
diff --git a/src/libmsc/gsm_04_08.c b/src/libmsc/gsm_04_08.c
index 0f52d09..d5cc212 100644
--- a/src/libmsc/gsm_04_08.c
+++ b/src/libmsc/gsm_04_08.c
@@ -1397,7 +1397,8 @@
 {
 	struct gsm48_hdr *gh;
 	uint8_t pdisc;
-	uint8_t n_sd, modulo, bin;
+	uint8_t n_sd, modulo;
+	int bin;
 
 	gh = msgb_l3(msg);
 	pdisc = gsm48_hdr_pdisc(gh);