Fix potential crash, fixes #12375: PATCH to fix help controller crash

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65371 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/fs_arc.cpp

@@ -358,7 +358,7 @@ wxFSFile* wxArchiveFSHandler::OpenFile(
         right = rightPart.GetFullPath(wxPATH_UNIX);
     }
 
-    if (right.GetChar(0) == wxT('/')) right = right.Mid(1);
+    if (right.Length() && right.GetChar(0) == wxT('/')) right = right.Mid(1);
 
     if (!m_cache)
         m_cache = new wxArchiveFSCache;