Fix potential segfault in dns_get_record()

If the remote sends us a packet with a malformed TXT record,
we could end up trying to over-consume the packet and wander
off into overruns.

ext/standard/dns.c

@@ -517,6 +517,10 @@ static u_char *php_parserr(u_char *cp, querybuf *answer, int type_to_fetch, int
 				
 				while (ll < dlen) {
 					n = cp[ll];
+					if ((ll + n) >= dlen) {
+						// Invalid chunk length, truncate
+						n = dlen - (ll + 1);
+					}
 					memcpy(tp + ll , cp + ll + 1, n);
 					add_next_index_stringl(entries, cp + ll + 1, n, 1);
 					ll = ll + n + 1;