Serial recovery with image state has null pointer usage

[nordicjm]

Commit d00b11d has introduced a regression, this added using the state to the maximum image size function, in serial recovery mode however this variable is NULL, which then causes a null pointer reference and undefined operation in swap using scratch, swap using move and swap using offset modes

[nordicjm]

@taltenbach

[nordicjm]

this is also broken in zephyr's v3.7 branch

[taltenbach]

Sorry for the regression, I wasn't expecting bootutil_img_validate to be called somewhere with a null state. At first sight, I think the proper solution would be to initialize a state (at least partially) in boot_serial. This seems to be already made in some places.
@nordicjm Are you already working on the issue? If not, I will try to create a PR in the coming week.

[nordicjm]

@nordicjm Are you already working on the issue? If not, I will try to create a PR in the coming week.

I am not