fix(api): open_win leak from naughty autocommands

Problem: error set by win_set_buf may leak if autocommands immediately close the
new window.

Solution: free the error set by win_set_buf. (prefer nvim_open_win's error as
it's more important and will cause 0 to be returned)

src/nvim/api/win_config.c

@@ -343,6 +343,7 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
     }
   }
   if (!tp) {
+    api_clear_error(err);  // may have been set by win_set_buf
     api_set_error(err, kErrorTypeException, "Window was closed immediately");
     goto cleanup;
   }

test/functional/api/window_spec.lua

@@ -2068,6 +2068,31 @@ describe('API/win', function()
       api.nvim_open_win(0, true, { split = 'below' })
       eq(11, api.nvim_win_get_height(0))
     end)
+
+    it('no leak when win_set_buf fails and window is closed immediately', function()
+      -- Following used to leak.
+      command('autocmd BufEnter * ++once quit! | throw 1337')
+      eq(
+        'Window was closed immediately',
+        pcall_err(
+          api.nvim_open_win,
+          api.nvim_create_buf(true, true),
+          true,
+          { relative = 'editor', width = 5, height = 5, row = 1, col = 1 }
+        )
+      )
+      -- If the window wasn't closed, still set errors from win_set_buf.
+      command('autocmd BufEnter * ++once throw 1337')
+      eq(
+        'BufEnter Autocommands for "*": 1337',
+        pcall_err(
+          api.nvim_open_win,
+          api.nvim_create_buf(true, true),
+          true,
+          { relative = 'editor', width = 5, height = 5, row = 1, col = 1 }
+        )
+      )
+    end)
   end)
 
   describe('set_config', function()