[addons][language] fix crash if selected language addon no more present

Previously, if the language addon was deleted for any reason, Kodi crashed on startup.

This adds a check that the add-on is actually there.

Here the backtrace about crash before:
```
\#0  CLangInfo::SetLanguage (this=0x55555ad37eb0, language="resource.language.de_de", reloadServices=false) at /home/alwin/Development/Kodi/kodi-Matrix/xbmc/LangInfo.cpp:670
\#1  0x0000555557ad51eb in CApplication::LoadLanguage (this=0x55555ad34c20, reload=false) at /home/alwin/Development/Kodi/kodi-Matrix/xbmc/Application.cpp:4915
\#2  0x0000555557abdbd0 in CApplication::Initialize (this=0x55555ad34c20) at /home/alwin/Development/Kodi/kodi-Matrix/xbmc/Application.cpp:737
\#3  0x0000555557635500 in XBMC_Run (renderGUI=true, params=...) at /home/alwin/Development/Kodi/kodi-Matrix/xbmc/platform/xbmc.cpp:46
\#4  0x0000555556e0b08c in main (argc=1, argv=0x7fffffffdf38) at /home/alwin/Development/Kodi/kodi-Matrix/xbmc/platform/posix/main.cpp:77
```

xbmc/LangInfo.cpp

@@ -667,7 +667,8 @@ bool CLangInfo::SetLanguage(std::string language /* = "" */, bool reloadServices
   // Find the chosen language add-on if it's enabled
   if (!addonMgr.GetAddon(language, addon, ADDON::ADDON_RESOURCE_LANGUAGE, true))
   {
-    if (addonMgr.IsAddonDisabled(addon->ID()) && !addonMgr.EnableAddon(addon->ID()))
+    if (!addonMgr.IsAddonInstalled(language) ||
+        (addonMgr.IsAddonDisabled(addon->ID()) && !addonMgr.EnableAddon(addon->ID())))
     {
       CLog::Log(LOGWARNING,
                 "CLangInfo::{}: could not find or enable language add-on '{}', loading default...",