misc: kgdbts: fix out-of-bounds access in function param_set_kgdbts_var

There is an out-of-bounds access to "config[len - 1]" array when the
variable "len" is zero.

See commit dada6a43 ("kgdboc: fix KASAN global-out-of-bounds bug
in param_set_kgdboc_var()") for details.

Signed-off-by: Young Xiao <YangX92@hotmail.com>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>