constrain gsm48_generate_mid() output array bounds

The longest BCd-digit type identity is the IMEISV with 16, so there's
no point in trying to parse up to 255 decimal digits, which will do
nothing but to overflow the caller-provided output buffer.

Let's also clearly define the required minimum size of the output
buffer and add a reltead #define for it.

Change-Id: Ic8488bc7f77dc9182e372741b88f0f06100dddc9
diff --git a/src/gb/gprs_bssgp_bss.c b/src/gb/gprs_bssgp_bss.c
index 487286c..77350e2 100644
--- a/src/gb/gprs_bssgp_bss.c
+++ b/src/gb/gprs_bssgp_bss.c
@@ -178,7 +178,7 @@
 				const char *imsi)
 {
 	struct msgb *msg = common_tx_radio_status(bctx);
-	uint8_t mi[10];
+	uint8_t mi[GSM48_MID_MAX_SIZE];
 	int imsi_len = gsm48_generate_mid_from_imsi(mi, imsi);
 
 	if (!msg)