crypto/x509/v3_utl.c: Fix IPv6 output in ipaddr_to_asc()

Fixes #12695

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/12696)

crypto/x509/v3_utl.c

@@ -978,7 +978,12 @@ int X509_check_ip_asc(X509 *x, const char *ipasc, unsigned int flags)
 
 char *ipaddr_to_asc(unsigned char *p, int len)
 {
+    /*
+     * 40 is enough space for the longest IPv6 address + nul terminator byte
+     * XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX\0
+     */
     char buf[40], *out;
+    int i = 0, remain = 0, bytes = 0;
 
     switch (len) {
     case 4: /* IPv4 */
@@ -986,11 +991,14 @@ char *ipaddr_to_asc(unsigned char *p, int len)
         break;
         /* TODO possibly combine with static i2r_address() in v3_addr.c */
     case 16: /* IPv6 */
-        for (out = buf; out < buf + 8 * 3; out += 3) {
-            BIO_snprintf(out, 3 + 1, "%X:", p[0] << 8 | p[1]);
+        for (out = buf, i = 8, remain = sizeof(buf);
+             i-- > 0 && bytes >= 0;
+             remain -= bytes, out += bytes) {
+            const char *template = (i > 0 ? "%X:" : "%X");
+
+            bytes = BIO_snprintf(out, remain, template, p[0] << 8 | p[1]);
             p += 2;
         }
-        out[-1] = '\0';
         break;
     default:
         BIO_snprintf(buf, sizeof(buf), "<invalid length=%d>", len);