jtag/drivers/ulink: fix clang static analyzer warning

jtag/drivers/ulink: fix clang static analyzer warning

scan-build-9:
Description: Access to field 'payload_in' results in a dereference
 of a null pointer (loaded from field 'queue_start')
File: src/jtag/drivers/ulink.c
Line: 2216

Set input/output_signals conditionally if ulink_append_get_signals_cmd()
and ulink_execute_queued_commands() returns no error.
Do not fail driver initialisation as the signals are only printed.

Change-Id: I6c842f0e9a604712abf7444a2fa95ba5810de1ff
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5520
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>