Compare Instants in Shard::latency

When calculating the latency between when a heartbeat was sent and a
heartbeat acknowledgement was received, ensure that the heartbeat
acknowledgement was received after the heartbeat was sent. This avoids a
panic. Otherwise, return None.

src/gateway/shard.rs

@@ -652,11 +652,13 @@ impl Shard {
     // Shamelessly stolen from brayzure's commit in eris:
     // <https://github.com/abalabahaha/eris/commit/0ce296ae9a542bcec0edf1c999ee2d9986bed5a6>
     pub fn latency(&self) -> Option<StdDuration> {
-        if let (Some(received), Some(sent)) = self.heartbeat_instants {
-            Some(sent - received)
-        } else {
-            None
+        if let (Some(sent), Some(received)) = self.heartbeat_instants {
+            if received > sent {
+                return Some(received - sent);
+            }
         }
+
+        None
     }
 
     #[cfg(feature = "voice")]