kconfig: nconf: Ensure null termination where strncpy is used

[ Upstream commit f468992936894c9ce3b1659cf38c230d33b77a16 ]

strncpy() does not guarantee null-termination if the source string is
longer than the destination buffer.

Ensure the buffer is explicitly null-terminated to prevent potential
string overflows or undefined behavior.

Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Nicolas Schier <n.schier@avm.de>
Acked-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>