throttle: Assert that bkt->max is valid in throttle_compute_wait()

If bkt->max == 0 and bkt->burst_length > 1 then we could have a
division by 0 in throttle_do_compute_wait(). That configuration is
however not permitted and is already detected by throttle_is_valid(),
but let's assert it in throttle_compute_wait() to make it explicit.

Found by Coverity (CID: 1381016).
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

util/throttle.c

@@ -124,6 +124,7 @@ int64_t throttle_compute_wait(LeakyBucket *bkt)
     /* If the main bucket is not full yet we still have to check the
      * burst bucket in order to enforce the burst limit */
     if (bkt->burst_length > 1) {
+        assert(bkt->max > 0); /* see throttle_is_valid() */
         extra = bkt->burst_level - burst_bucket_size;
         if (extra > 0) {
             return throttle_do_compute_wait(bkt->max, extra);