jtag: opendous: fix tap buffer overflow

Appending bits to TAP buffer doesn't check if there's enough space left.
This patch adds this check to fix TAP overflow error.

Change-Id: If80d5ab4a24983ad24f3cab31f9676d1590ebf5d
Signed-off-by: Stefan Mahr <stefan.mahr@sphairon.com>
Reviewed-on: http://openocd.zylin.com/1216
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>