Fix: use of file pointer after fclose()

See fclose(3). Further access to out_fp after both success and error,
even through another bt_close_memstream(), results in undefined
behavior. Nullify out_fp to ensure we don't fclose it twice on error.

Found by Coverity:

CID 1376182 (#1 of 1): Use after close (USE_AFTER_FREE).
use_closed_file: Calling bt_close_memstream uses file handle out_fp
after closing it.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>