contrib/bash_completion.d: squelch FreeBSD seq when first < last

With seq x -1 z and x is less than z FreeBSD seq will print the error:

$ seq 1 -1 2
seq: needs positive increment

Hide this error. Alternatively $COMP_CWORD could be checked for < 2.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Derek Schrock <dereks@lifeofadishwasher.com>
Closes #16234