network: eliminate potential memory leak on parse failure

network: eliminate potential memory leak on parse failure

While the first encountered dns host record is being parsed, it's
possible for virNetworkDef::hosts to point to memory that has been
allocated, but virNetworkDef::nhosts to still be 0. If there is a
failure during that time, virNetworkDef::hosts will be leaked.

Although this isn't currently the case for virNetworkDef::txtrecords,
it could become that way through future re-factoring, and it hurts
nothing to restructure the freeing of txtrecord data to match that of
hosts data.