kconfig: mconf,nconf: remove unneeded '\0' termination after snprintf()

kconfig: mconf,nconf: remove unneeded '\0' termination after snprintf()

snprintf() always terminates the destination buffer with '\0' even if
the buffer is not long enough. (In this case, the last element of the
buffer becomes '\0'.)

The explicit termination is unneeded.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>