`max_target_size` is incorrectly determined when loading delegated targets

[ecpullen]

When trying to load a TUF repo with delegated targets, if a delegated role has a longer configuration file than the targets.json it will be unable to be loaded.

In the load_targets function, the max_targets_length variable is changed to the actual length of the targets metadata -

tough/tough/src/lib.rs

Line 1200 in c0d27ad

let (max_targets_size, specifier) = match targets_meta.length {

That updated max_targets_size is then passed when loading delegated roles -

tough/tough/src/lib.rs

Line 1266 in c0d27ad

max_targets_size,