winhelp: do not create popup window if file not found

[Kirill K. Smirnov]

ChangeLog:
  do not create popup window if file not found

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diff --git a/programs/winhelp/winhelp.c b/programs/winhelp/winhelp.c
index 389ba5a..f7c0ae3 100644
--- a/programs/winhelp/winhelp.c
+++ b/programs/winhelp/winhelp.c
@@ -1195,7 +1195,7 @@ static LRESULT CALLBACK WINHELP_TextWndP
                 break;
             case hlp_link_popup:
                 hlpfile = WINHELP_LookupHelpFile(part->link->lpszString);
-                WINHELP_CreateHelpWindowByHash(hlpfile, part->link->lHash, 
+                if (hlpfile) WINHELP_CreateHelpWindowByHash(hlpfile, part->link->lHash, 
                                                WINHELP_GetPopupWindowInfo(hlpfile, hWnd, &mouse),
                                                SW_NORMAL);
                 break;