Fix closing bug in manifold.stream/concat

When `in` is closed and `out` is also closed by the time the chained handler runs, closing `s'`
would fail because it's `::none`. This non-deterministic case was occasionally observed with
`manifold.stream-test/test-cleanup` failing due to a leaked error deferred like so:

    WARNING: unconsumed deferred in error state, make sure you're using `catch`.
    java.lang.ClassCastException: clojure.lang.Keyword cannot be cast to manifold.stream.core.IEventStream
            at manifold.stream$concat$this__3189__auto____14958$fn__14959$fn__14960.invoke(stream.clj:819)
            at manifold.deferred$eval3018$chain_SINGLEQUOTE____3039.invoke(deferred.clj:904)
            at manifold.stream$concat$this__3189__auto____14958$fn__14959.invoke(stream.clj:816)
            at manifold.stream$concat$this__3189__auto____14958.invoke(stream.clj:815)
            at clojure.lang.AFn.applyToHelper(AFn.java:154)
            at clojure.lang.AFn.applyTo(AFn.java:144)
            at clojure.core$apply.invokeStatic(core.clj:669)
            at clojure.core$apply.invoke(core.clj:662)
            at manifold.stream$concat$this__3189__auto____14958$fn__14984.invoke(stream.clj:815)

Author: DerGuteMoritz

src/manifold/stream.clj

@@ -816,14 +816,14 @@
       (d/chain' (take! in ::none)
                 (fn [s']
                   (cond
-                    (closed? out)
-                    (close! s')
-
                     (identical? ::none s')
                     (do
                       (close! out)
                       s')
 
+                    (closed? out)
+                    (close! s')
+
                     :else
                     (d/loop []
                       (d/chain' (take! s' ::none)