box/console: fix abnormal exit after unknown command

When invalid command is passed we should send an error message to a
client. Instead a nil dereference occurs that causes abnormal exit of a
console.

This is the regression from 96dbc49d
('box/console: Refactor command handling').

Reported-by: Mergen Imeev <imeevma@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>

src/box/lua/console.lua

@@ -323,7 +323,7 @@ local function preprocess(storage, line)
     end
     if items == nil then
         local msg = "Invalid command \\%s. Type \\help for help."
-        return format(false, msg:format(items[1]))
+        return format(false, msg:format(line))
     end
     return operators[items[1]](storage, unpack(items))
 end

test/app-tap/console.test.lua

@@ -21,7 +21,7 @@ local EOL = "\n...\n"
 
 test = tap.test("console")
 
-test:plan(72)
+test:plan(73)
 
 -- Start console and connect to it
 local server = console.listen(CONSOLE_SOCKET)
@@ -292,6 +292,20 @@ client:write("box.session.type();\n")
 test:is(yaml.decode(client:read(EOL))[1], "console", "session type")
 client:close()
 
+--
+-- An unknown backslash started command causes abnormal exit of
+-- a console.
+--
+local cmd = '\\unknown_command'
+local exp_res = {error = string.format(
+    'Invalid command %s. Type \\help for help.', cmd)}
+client = socket.tcp_connect("unix/", CONSOLE_SOCKET)
+client:read(128)
+client:write(('%s\n'):format(cmd))
+local res = yaml.decode(client:read(EOL))[1]
+test:is_deeply(res, exp_res, 'unknown command')
+client:close()
+
 server:close()
 
 box.schema.user.drop('test')