Bootutil: always initialise the size returned by boot_util_image_size()

Assert on the pointer validity in debug builds and make sure that
an initialised value of 0 is always returned even in case of errors

Change-Id: Ia0546941296fc9741c1b20955d7c7a0d47c7b2e1
Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>