findmnt: always zero-terminate SOURCES data

findmnt: always zero-terminate SOURCES data

libsmartcols expects it's data fields to be zero terminated.
See the call to strlen() in scols_column_greatest_wrap().
ul_buffer however does not guarantee that termination,
ul_buffer_append_strings() discard the zero-termination.

Always zero-terminate in get_data_col_sources() and drop the now
unnecessary variable "i".

Closes: https://github.com/util-linux/util-linux/issues/2980
Signed-off-by: Thomas Weißschuh <thomas@t-8ch.de>