Protect WDML_GetConv against null handles.

diff --git a/dlls/user/dde/misc.c b/dlls/user/dde/misc.c
index 6c262ac..51e24bf 100644
--- a/dlls/user/dde/misc.c
+++ b/dlls/user/dde/misc.c
@@ -1902,6 +1902,7 @@
     WDML_CONV*	pConv = (WDML_CONV*)hConv;
 
     /* FIXME: should do better checking */
+    if (pConv == NULL) return NULL;
 
     if (checkConnected && !(pConv->wStatus & ST_CONNECTED))
     {