tail: avoid misleading diagnostic upon fstat failure

tail: avoid misleading diagnostic upon fstat failure

* src/tail.c (check_fspec): Save fstat-induced errno *before*
calling close_fd, not after.  Otherwise, the close could well
clobber the global errno, making tail print an invalid diagnostic.
This could happen only with tail -f, and even then, only when
a valid file descriptor were to provoke fstat failure.