msctf: ITfDocumentMgr::GetBase returns the top if there is only one context pushed.

dlls/msctf/documentmgr.c

@@ -199,14 +199,21 @@ static HRESULT WINAPI DocumentMgr_GetTop(ITfDocumentMgr *iface, ITfContext **ppi
 static HRESULT WINAPI DocumentMgr_GetBase(ITfDocumentMgr *iface, ITfContext **ppic)
 {
     DocumentMgr *This = (DocumentMgr *)iface;
+    ITfContext *tgt;
+
     TRACE("(%p)\n",This);
     if (!ppic)
         return E_INVALIDARG;
 
     if (This->contextStack[1])
-        ITfContext_AddRef(This->contextStack[1]);
+        tgt = This->contextStack[1];
+    else
+        tgt = This->contextStack[0];
+
+    if (tgt)
+        ITfContext_AddRef(tgt);
 
-    *ppic = This->contextStack[1];
+    *ppic = tgt;
 
     return S_OK;
 }