backtrace: fix off-by-one on string output

sizeof("foo") includes the trailing null byte, so all the output had
nulls through it. Most terminals quietly ignore it, but it makes some
tools misdetect file types and other annoyances.

Easy fix: subtract 1.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16862