Fix up r300385

I accidentally glossed over the fact that tmp is manipulated via strchr, so
if we tried to free tmp after r300385, it would have crashed.

Create a separate pointer (tmp2) to track the original allocation of tmp,
and free tmp2 if p->nc_lookups can't be malloced

MFC after: 4 days
X-MFC with: r300385
Reported by: Coverity
CID: 1356026
Sponsored by: EMC / Isilon Storage Division