BugLink: https://bugs.launchpad.net/bugs/1836666

[ Upstream commit b281218a ]

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>