Fix bug with #_ reader macro used on the last form

[ingydotnet]

Before:

$ bin/linux_amd64/glj -e '(prn 1) #_(prn 2) (prn 3)'
1
3
$ bin/linux_amd64/glj -e '(prn 1) (prn 2) #_(prn 3)'
1
2
2025/08/15 13:06:23 <unknown-file>:1:25: error reading input: EOF
$ 

After:

$ bin/linux_amd64/glj -e '(prn 1) (prn 2) #_(prn 3)'
1
2
$ 

[jfhamlin]

I happened to get to this when working on reader conditionals here: #109
A complete fix was a little more involved! Thanks for taking a crack at it!