gdb server: Fix buffer overrun - sprintf appends a terminating null to the data which was overrunning the supplied buffer.

Fixes regression introduced in commit 07dcd5648d146d38f9ffa619f0737587e592d0b6

Signed-off-by: Evan Hunter <ehunter@broadcom.com>
Change-Id: Iec64233c0da5a044fb984c4b1803309cb636efe9
Reviewed-on: http://openocd.zylin.com/1312
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>