allow deferred close of nil stream (#20706)

* allow deferred close of nil stream

* improve example

lib/pure/streams.nim

@@ -173,10 +173,20 @@ proc close*(s: Stream) =
   ## See also:
   ## * `flush proc <#flush,Stream>`_
   runnableExamples:
-    var strm = newStringStream("The first line\nthe second line\nthe third line")
+    block:
+      let strm = newStringStream("The first line\nthe second line\nthe third line")
       ## do something...
       strm.close()
-  if not isNil(s.closeImpl): s.closeImpl(s)
+      
+    block:
+      let strm = newFileStream("amissingfile.txt")
+      # deferring works even if newFileStream fails
+      defer: strm.close()
+      if not isNil(strm):
+        ## do something...
+
+  if not isNil(s) and not isNil(s.closeImpl):
+    s.closeImpl(s)
 
 proc atEnd*(s: Stream): bool =
   ## Checks if more data can be read from `s`. Returns ``true`` if all data has

tests/stdlib/tstreams.nim

@@ -50,6 +50,12 @@ block tstreams3:
       echo line
     s.close
 
+
+block:
+  let fs = newFileStream("amissingfile.txt")
+  defer: fs.close()
+  doAssert isNil(fs)
+
 # bug #12410
 
 var a = newStringStream "hehohihahuhyh"