kconfig: fix possible buffer overflow

Buffer 'new_argv' is accessed without bound check after accessing with
bound check via 'new_argc' index.

Fixes: e298f3b4 ("kconfig: add built-in function support")
Co-developed-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>