[settings] Avoid overwriting the start of .text in fetch_string_setting()

[settings] Avoid overwriting the start of .text in fetch_string_setting()

fetch_string_setting() was subtracting one from the length of the
to-be-NUL-terminated buffer in order to obtain the length of the
unterminated buffer to be passed to fetch_setting().  This works
extremely well unless the length of the to-be-NUL-terminated buffer is
zero, at which point we end up giving fetch_setting() a buffer of
length -1UL, thereby inviting it to overwrite as much memory as it
wants...