core: don't skip entries in _ostree_validate_structureof_xattrs

core: don't skip entries in _ostree_validate_structureof_xattrs

the for loop already advances the index, but the body incremented it a
second time, so only even-indexed xattrs were examined and the empty-name,
duplicate-name and sort-order checks skipped half the array. a crafted
dirmeta from a remote could hide an unsorted or duplicate xattr in an odd
slot and still pass ostree_validate_structureof_dirmeta.