ldb:kv_index: help static analysers to not worry (CID 1615192)

ldb:kv_index: help static analysers to not worry (CID 1615192)

The point of this realloc is that we are not using this array, but
keeping it around to remain a node the talloc tree. We'd prefer to
reduce it to nothing.

Coverity rightly spotted that it was reallocing an array of `struct
ldb_val` to an array of `struct ldb_val *`, which has a different size
and all. But it doesn't matter in this case, because we will never use
it.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15590

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jennifer Sutton <josutton@catalyst.net.nz>