tail: avoid read-beyond-end-of-buffer error

tail: avoid read-beyond-end-of-buffer error

* src/tail.c (tail_forever_inotify): Do not use f[i] in a context
where i may be larger than the largest valid index.  In the final
"if" clause in which we'd remove an inotify watch, we might have
used f[n_files].  Use fspec instead, since it is guaranteed to
be defined.