warning fix: add self-consitency check to remove warning

warning fix: add self-consitency check to remove warning

verify promise of code that more code can be pasted with an
assert at the end condition of the code passage that builds
string.

Change-Id: I76a4e5f91b9142fff932e1493cb43c29eb6a0f80
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/143
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>