TOOLS: Fix minor memory leak in sss_colondb_writeline
The variable line was initialized to NULL.
The we created temporary context tmp_ctx.
We use talloc_asprintf_append to append string to line which is initially
NULL and therefore new context which was not connected to tmp_ctx.
man 3 talloc_string -> talloc_asprintf_append
Reviewed-by: Petr Cech <pcech@redhat.com>