Guard against nil pointer dereference

It is possible for timer to be nil if maxAttempts is zero.

psiphon/common/resolver/resolver.go

@@ -892,7 +892,9 @@ func (r *Resolver) ResolveIP(
 		}
 	}
 
-	timer.Stop()
+	if timer != nil {
+		timer.Stop()
+	}
 
 	// Interrupt all workers.
 	cancelFunc()