settings: zms: fix out-of-bounds null terminator write

Previously, zms_read() could fill the buffer up to sizeof(rdname),
leaving no space for the null terminator, which could cause an
out-of-bounds write.

This change reduces the read size to sizeof(rdname) - 1 and appends
a '\0' manually, ensuring the buffer is always null-terminated safely.

Fixes: CID 516244
Fixes: #90533

Signed-off-by: sudarsan N <sudarsansamy2002@gmail.com>