vlr.c: fix condition to check MSISDN presence

msisdn_enc is a buffer, its address is always != 0

Change-Id: Ib2294d2cd339c36df599d7d134f979a572ac308a
diff --git a/src/libvlr/vlr.c b/src/libvlr/vlr.c
index f644df8..b164fd8 100644
--- a/src/libvlr/vlr.c
+++ b/src/libvlr/vlr.c
@@ -784,7 +784,7 @@
 	unsigned idx;
 	int rc;
 
-	if (gsup_msg->msisdn_enc) {//FIXME: vlr_subscr_set_msisdn()?
+	if (gsup_msg->msisdn_enc_len) {//FIXME: vlr_subscr_set_msisdn()?
 		gsm48_decode_bcd_number2(vsub->msisdn, sizeof(vsub->msisdn),
 					 gsup_msg->msisdn_enc,
 					 gsup_msg->msisdn_enc_len, 0);