column: segmentation fault on invalid unicode input passed to -s option

The function mbs_to_wcs() returns NULL on invalid UTF.

Fixes: https://github.com/karelzak/util-linux/issues/1425
Signed-off-by: Karel Zak <kzak@redhat.com>

text-utils/column.c

@@ -814,6 +814,8 @@ int main(int argc, char **argv)
 		case 's':
 			free(ctl.input_separator);
 			ctl.input_separator = mbs_to_wcs(optarg);
+			if (!ctl.input_separator)
+				err(EXIT_FAILURE, _("failed to use input separator"));
 			ctl.greedy = 0;
 			break;
 		case 'T':