tail: don't call fstat on an uninitialized FD

tail: don't call fstat on an uninitialized FD

This bug showed up via valgrind as a "Conditional jump or move
depends on uninitialized value(s)" error.
* src/tail.c (ignore_fifo_and_pipe): New function.
(main): Use it only when tailing forever.
The code to compute n_viable and mark some F[i] as ignored would call
isapipe on an uninitialized file descriptor.  But n_viable and those
.ignored marks are useful/used only when tailing forever.  This bug
was introduced via commit f0ff8c73 (7.6), "tail: make the new
piped-stdin test as portable as the old one".
* NEWS (Bug fixes): Mention it.