protect UNO EnvStack's s_setCurrent() against unknown threadid

untracked threadIds have been observed and can result in crashes.
s_getCurrent() already checks them and s_setCurrent() does it now.

cppu/source/uno/EnvStack.cxx

@@ -76,10 +76,10 @@ static void s_setCurrent(uno_Environment * pEnv)
     osl::MutexGuard guard(s_threadMap_mutex);
     if (pEnv)
         s_threadMap[threadId] = pEnv;
-
     else
     {
         ThreadMap::iterator iEnv = s_threadMap.find(threadId);
+        if( iEnv != s_threadMap.end())
             s_threadMap.erase(iEnv);
     }
 }