fix(channel): crash on exit after closing v:stderr channel (#38754)

Problem:  Crash on exit after closing v:stderr channel when piping
          to stdin.
Solution: Reopen stderr as /dev/null or NUL instead of closing it.
          This also avoids writing to an related file if one is opened
          after closing v:stderr.
(cherry picked from commit e20c4ea966)

src/nvim/channel.c

@@ -167,7 +167,13 @@ bool channel_close(uint64_t id, ChannelPart part, const char **error)
       chan->stream.err.closed = true;
       // Don't close on exit, in case late error messages
       if (!exiting) {
-        fclose(stderr);
+        // Don't close the file descriptor, as that may cause later writes to stderr
+        // to go to an unrelated file. Redirect it to NUL or /dev/null instead.
+#ifdef MSWIN
+        freopen("NUL:", "w", stderr);
+#else
+        freopen("/dev/null", "w", stderr);
+#endif
       }
       channel_decref(chan);
     }

test/functional/core/exit_spec.lua

@@ -119,12 +119,12 @@ describe(':cquit', function()
   end)
 end)
 
-describe('no crash after :quit non-last window during exit', function()
+describe('when piping to stdin, no crash during exit', function()
   before_each(function()
     n.clear()
   end)
 
-  it('in vim.schedule() callback and when piping to stdin #14379', function()
+  it('after :quit non-last window in vim.schedule() callback #14379', function()
     n.fn.system({
       n.nvim_prog,
       '-es',
@@ -135,7 +135,7 @@ describe('no crash after :quit non-last window during exit', function()
     eq(0, n.api.nvim_get_vvar('shell_error'))
   end)
 
-  it('in vim.defer_fn() callback and when piping to stdin #14379', function()
+  it('after :quit non-last window in vim.defer_fn() callback #14379', function()
     n.fn.system({
       n.nvim_prog,
       '-es',
@@ -145,4 +145,15 @@ describe('no crash after :quit non-last window during exit', function()
     }, '')
     eq(0, n.api.nvim_get_vvar('shell_error'))
   end)
+
+  it('after closing v:stderr channel', function()
+    n.fn.system({
+      n.nvim_prog,
+      '-es',
+      '--cmd',
+      'call chanclose(v:stderr)',
+      '+quit',
+    }, '')
+    eq(0, n.api.nvim_get_vvar('shell_error'))
+  end)
 end)