Limit 'NadcsPerImage' to be at least 1

[Stockless]

As "NadcsPerImage" shouldn't ever be less than 1, it gets limited to be the at least 1. This implementation avoids a possible division by 0 in ISMRMRD.jl, Solving the Issue #409 :

if scan_counter % NadcsPerImage == 0 #For now only Nz is considered
    nz += 1 #another image
    scan_counter = 0 #reset counter
end

[cncastillo]

This needs to be done from this repo, not a fork.