Check if watcher has already been cancelled

This seems to only happen in 7.1 for some reason… will perhaps need to dig further into why.

lib/Context/Parallel.php

@@ -283,7 +283,7 @@ final class Parallel implements Context
 
         unset(self::$futures[$this->id], self::$channels[$this->id]);
 
-        if (empty(self::$futures)) {
+        if (empty(self::$futures) && self::$watcher !== null) {
             Loop::cancel(self::$watcher);
             self::$watcher = null;
         }