Check for end-of-buffer

Enabling judge-indent in a buffer where the last line starts with an
indentation immediately followed by EOB with no newline currently fails:

    $ echo -n '\n\t' >foo
    $ emacs -batch -Q -l judge-indent.el ./foo -f judge-indent-mode
    Loading vc-git...
    Wrong type argument: characterp, nil

This patch makes sure that `char-equal` is not called with an argument
of `nil`.