sgsn_vty: Fix uninitialized variable in the gprs_apn2str method

This is fixing a GCC and Coverity warning:

GCC:
sgsn_vty.c: In function ‘vty_dump_pdp’:
sgsn_vty.c:64:5: warning: ‘i’ may be used uninitialized in this function [-Wmaybe-uninitialized]
sgsn_vty.c:49:15: note: ‘i’ was declared here

Coverity: CID 1040706

openbsc/src/gprs/sgsn_vty.c

@@ -46,7 +46,7 @@ static struct sgsn_config *g_cfg = NULL;
 static char *gprs_apn2str(uint8_t *apn, unsigned int len)
 {
 	static char apnbuf[GSM48_MAX_APN_LEN+1];
-	unsigned int i;
+	unsigned int i = 0;
 
 	if (!apn)
 		return "";