nis_rpcent: don't leak resultbuf from yp_first(..)/yp_next(..)

If the buffer couldn't be adequately resized to accomodate an additional "\n",
it would leak resultbuf by breaking from the loop early

MFC after: 2 weeks
Reported by: Coverity
CID: 1016702
Sponsored by: EMC / Isilon Storage Division