lib/display_options: avoid illegal memory access

lib/display_options: avoid illegal memory access

display_options_get_banner_priv() overwrites bytes before the start of the
buffer if the buffer size is less then 3. This case occurs in the Sandbox
when executing the `ut_print` command.

Correctly handle small buffer sizes. Adjust the print unit test to catch
when bytes before the buffer are overwritten.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>