Fix potentially-uninitialized critical section in Win32 DLL builds

If non-parser parts of libxml (e.g. xmlwriter) are used before a parser,
xmlOnceInit may have run (e.g. via the many paths to xmlGetGlobalState),
but not xmlInitThreads (which is called only by xmlInitParser)

Once globalkey != TLS_OUT_OF_INDEXES (which can happen in many ways),
DLLMAIN(DLL_THREAD_DETACH) may attempt to lock cleanup_helpers_cs
before it is valid. This may happen even if the thread whose exit
is triggering DllMain is from code which is not linked to libxml.

globalkey and cleanup_helpers_cs should be initialized together,
with cleanup_helpers_cs initialized first and deleted last.

threads.c

@@ -885,8 +885,6 @@ xmlInitThreads(void)
         }
     }
 #endif /* XML_PTHREAD_WEAK */
-#elif defined(HAVE_WIN32_THREADS) && !defined(HAVE_COMPILER_TLS) && (!defined(LIBXML_STATIC) || defined(LIBXML_STATIC_FOR_DLL))
-    InitializeCriticalSection(&cleanup_helpers_cs);
 #endif
 }
 
@@ -958,6 +956,9 @@ xmlOnceInit(void)
     if (!run_once.done) {
         if (InterlockedIncrement(&run_once.control) == 1) {
 #if !defined(HAVE_COMPILER_TLS)
+#if !defined(LIBXML_STATIC) || defined(LIBXML_STATIC_FOR_DLL)
+            InitializeCriticalSection(&cleanup_helpers_cs);
+#endif
             globalkey = TlsAlloc();
 #endif
             mainthread = GetCurrentThreadId();