Clipboard: [windows] Fixed not closing win32 clipboard on early return. (#1264)

imgui.cpp

@@ -9655,7 +9655,10 @@ static const char* GetClipboardTextFn_DefaultImpl(void*)
         return NULL;
     HANDLE wbuf_handle = GetClipboardData(CF_UNICODETEXT);
     if (wbuf_handle == NULL)
+    {
+        CloseClipboard();
         return NULL;
+    }
     if (ImWchar* wbuf_global = (ImWchar*)GlobalLock(wbuf_handle))
     {
         int buf_len = ImTextCountUtf8BytesFromStr(wbuf_global, NULL) + 1;