Avoid crash when checking property link dependencies without link info

Teach the isLinkDependentProperty helper added by commit e9879910 (Make
INTERFACE determined properties readable in generator expressions,
2013-01-19) to return early if no link information is available.

Source/cmTarget.cxx

@@ -4839,6 +4839,10 @@ bool isLinkDependentProperty(cmTarget *tgt, const std::string &p,
                              const char *config)
 {
   cmComputeLinkInformation *info = tgt->GetLinkInformation(config);
+  if(!info)
+    {
+    return false;
+    }
 
   const cmComputeLinkInformation::ItemVector &deps = info->GetItems();