column: fix leading space characters bug

column: fix leading space characters bug

The bug has been introduced during column(1) rewrite. The function
read_input() need to skip leading space only temporary to detect empty
lines, but the rest of the code has to use the original buffer (line).
I've tried to fix one of the symptoms by 5c7b67fbbf41c973ca8d49b1e8bdba22dbb917aa
(alter), but this solution is unnecessary and too complex.

Changes:

* don't ignore leading space
* remove unnecessary stuff introduced by 5c7b67fbbf41c973ca8d49b1e8bdba22dbb917aa
* fix regression test with incorrect separator

Addresses: https://github.com/karelzak/util-linux/issues/575
Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1560283
Signed-off-by: Karel Zak <kzak@redhat.com>