Avoid crash when checking property compatibility without link info

Teach the compatibility check added by commit 042ecf04 (Add API to
calculate link-interface-dependent bool properties or error, 2013-01-06)
to return early if no link information is available.  This avoids
crashing in a case that should fail with an error message.

Source/cmTarget.cxx

@@ -4699,6 +4699,10 @@ PropertyType checkInterfacePropertyCompatibility(cmTarget *tgt,
       || (!impliedByUse && !explicitlySet));
 
   cmComputeLinkInformation *info = tgt->GetLinkInformation(config);
+  if(!info)
+    {
+    return propContent;
+    }
   const cmComputeLinkInformation::ItemVector &deps = info->GetItems();
   bool propInitialized = explicitlySet;