tree-cache: don't error out on a childless invalidated entry

tree-cache: don't error out on a childless invalidated entry

The code used to assume that there had to be data after the newline in
a tree cache extension entry. This isn't true for a childless
invalidated entry if it's the last one, as there won't be any children
nor a hash to take up space.

Adapt the off-by-one comparison to also work in this case. Fixes #633.