802.15.4: fix various length checks and other issues.

802.15.4: fix various length checks and other issues.

Fix some length checks done before subtracting from a length value to
test whether the length value is < the full amount to be subtracted from
it, not just part of that amount.

Add some such checks where they were needed but not present.

Make some values unsigned if they're never negative.

This should fix some issues detected by undefined-behavior sanitizers.