Use `checked_sub` instead of an unchecked subtraction

https://rust-lang.github.io/rust-clippy/master/index.html#unchecked_duration_subtraction

acmed/src/endpoint.rs

@@ -101,7 +101,8 @@ impl RateLimit {
 
 	fn request_allowed(&self) -> bool {
 		for (max_allowed, duration) in self.limits.iter() {
-			let max_date = Instant::now() - *duration;
+			match Instant::now().checked_sub(*duration) {
+				Some(max_date) => {
 					let nb_req = self
 						.query_log
 						.iter()
@@ -111,13 +112,19 @@ impl RateLimit {
 						return false;
 					}
 				}
+				None => {
+					return false;
+				}
+			};
+		}
 		true
 	}
 
 	fn prune_log(&mut self) {
 		if let Some((_, max_limit)) = self.limits.first() {
-			let prune_date = Instant::now() - *max_limit;
+			if let Some(prune_date) = Instant::now().checked_sub(*max_limit) {
 				self.query_log.retain(move |&d| d > prune_date);
 			}
 		}
+	}
 }