prevent exception evaluating recall tooltip for recall chain without a remember

[Jamiras]

Fixes an error showing the tooltip for a {recall} that points at a Remembered value that is using a {recall} that isn't pointing at anything.

When a {recall} doesn't point at anything, the tooltip shows [Invalid recall], but when the missing Remember is further up the chain, an exception was occurring. Now, the broken Remember will report 0 as being remembered and that will appear in the tooltip for the latter nodes.