sync: use asserts in channel code only when `-d debug` is passed

vlib/sync/bench/channel_bench_v.v

@@ -41,7 +41,9 @@ fn main() {
 		go do_rec(ch, resch, n)
 		no -= n
 	}
+	$if debug {
 		assert no == 0
+	}
 	no = nobj
 	for i in 0 .. nsend {
 		n := no / (nsend - i)

vlib/sync/channels.c.v

@@ -547,8 +547,10 @@ fn (mut ch Channel) try_pop_priv(dest voidptr, no_block bool) ChanState {
 //               -2 if all channels are closed
 
 pub fn channel_select(mut channels []&Channel, dir []Direction, mut objrefs []voidptr, timeout time.Duration) int {
+	$if debug {
 		assert channels.len == dir.len
 		assert dir.len == objrefs.len
+	}
 	mut subscr := []Subscription{len: channels.len}
 	mut sem := unsafe { Semaphore{} }
 	sem.init(0)