core/unit: delay creating a stack variable until after length has been checked

core/unit: delay creating a stack variable until after length has been checked

path_is_normalized() will reject paths longer than 4095 bytes, so it's better
to not create a stack variable of unbounded size, but instead do the check first
and only then do that allocation.

Also use _cleanup_ to make things a bit shorter.

https://oss-fuzz.com/v2/issue/5424177403133952/7000