Fix ynh_print_OFF when set -x is used in other helpers #733
As reported by @Josue-T, ynh_print_OFF doesn't work when used before a helper using getopts, because the helper getopts use set -x.
Redirect BASH_XTRACEFD, the file descriptor for xtrace, to /dev/null to get rid of the trace.
Tested on leed.
How to test