1) Fix longstanding math bug with denominator > 1 (wrong probability).

1) Fix longstanding math bug with denominator > 1 (wrong probability).
Test case:
	random -f some_small_file 10000
(in most cases must be no output)
2) Prevent number of lines > RANDOM_MAX (overflow or nothing may be
choosed) with EFBIG err()
3) After line is found, terminate list loop for -U case too,
since nothing to do in the rest of the loop left.