libdtrace: Don't try to unput a nul terminator

[markj]

Summary

input() returns 0 to indicate EOF conditions, not EOF.

This fixes most instances of unknown argument types showing up in dtrace
-lv output. The rest seem to be related to duplicate type names. For
instance, if I run 'dtrace -lv -n fbt:::entry', on my system dtrace
fails to resolve the first parameter to
fbt::vm_pageout_reinsert_inactive:entry.
If I list the parameters for that probe specifically, it works, so it's
not as severe a bug.

[rstone]

Thanks for looking into this for me.

[markj]

This was fixed a different way by commit 6b7e592c215f.