The sd and od drivers didn't check for negative block numbers (like wd.c

The sd and od drivers didn't check for negative block numbers (like wd.c
does) before calling dscheck().  dscheck() doesn't appreciate this and
calls Debugger() and returns without setting bp->b_error.

This can happen when there is a casting error and offsets > 2G are
converted to negative off_t's in the disk tools.  (dumpfs used to do this).