jtag/drivers/ulink: fix clang static analyzer warnings

jtag/drivers/ulink: fix clang static analyzer warnings

scan-build-9:
Description: Potential leak of memory pointed to by 'cmd'
File: /home/vanekt/openocd/scanbuild9/../src/jtag/drivers/ulink.c
Line: 1075

Description: Potential leak of memory pointed to by 'cmd'
File: /home/vanekt/openocd/scanbuild9/../src/jtag/drivers/ulink.c
Line: 1275

ulink_append_xxx_cmd() functions allocate memory for cmd
and then call ulink_allocate_payload(), which allocates cmd->payload_out
or cmd->payload_in.

ulink_append_queue() checks the size of queue and if the new payload
does not fit, calls ulink_execute_queued_commands() and then
ulink_post_process_queue(). If any of these two fails, an error is
returned, allocated cmd struct leaks and the queue is left in an undefined
state.

Change ulink_append_queue() flow to proceed to appending cmd to the queue
even in the case of fail in previous ulink_execute_queued_commands()
or ulink_post_process_queue(). In case of fail then clear the queue
including the last appended cmd.

Change-Id: I967c07af19e9020c93bcb4ef403cf1f557dd1db1
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5370
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>