xsvf: fix clang warning

clang reports 'Function call argument is an uninitialized value'.

Change-Id: I50f4a7932b59930a5f1e3ece70b12c59e85ea3c6
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/360


Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>