Fix case where interval overflows (thanks jackeri)

[17:58] <Jacker> hey, you might be interested in checking out this
https://github.com/etlegacy/etlegacy/commit/4da5a397b5994bfe5fddb9dad35bef5ddbea64c9#diff-acaedc9d8b492f9af8966ae68597392cR615
[17:58] <Jacker> its related to the ddos protection code you wrote
[17:59] <Jacker> in continuation to:
ab9b08e584
[17:59] <Jacker> in a case if the client has in the past connected to
the server days/weeks earlier and time wraps the client wont be able to
connect
[18:00] <Jacker> since in that case if the bucket of that clients ip
still exists it wont get checked correctly

code/server/sv_main.c

@@ -487,7 +487,7 @@ qboolean SVC_RateLimit( leakyBucket_t *bucket, int burst, int period ) {
 		int expired = interval / period;
 		int expiredRemainder = interval % period;
 
-		if ( expired > bucket->burst ) {
+		if ( expired > bucket->burst || interval < 0 ) {
 			bucket->burst = 0;
 			bucket->lastTime = now;
 		} else {