Add checks for a negative or too-long snapshot length.

Add checks for a negative or too-long snapshot length.

The latter check should prevent overflows on ILP32 platforms, although
they're unlikely to happen in practice, given the limits we impose on
snapshot lengths.  That should fix Coverity CID 1420971.

The former check means we can get away with casting the snapshot length
to unsigned int, suppressing a signed vs. unsigned comparison warning.
This should never fail in practice.