semihosting: fix accessing memory outside the bounds of the fn array

semihosting: fix accessing memory outside the bounds of the fn array

There is an accsess to wrong index, when arm semihosting_basedir
command not used or basedir set to empty string.

Signed-off-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Change-Id: I3afa049d74b30496f5c03ba4ef67431784f81bdc
Fixes: ce5027ab019a ("semihosting: add semihosting_basedir command")
Reviewed-on: https://review.openocd.org/c/openocd/+/7005
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>