Fix an off-by-one bug in string/array handling which lead to memory overwrite
and follow-up assertion errors on at least ARM after r282257,
with nvp_magic being 0x6e7600:
Assertion failed: ((nvp)->nvp_magic == 0x6e7670), function nvpair_name, file .../subr_nvpair.c, line 713.

Sponsored by: DARPA/AFRL