Properly handle updates of variably-sized SA entries.
During the update process in sa_modify_attrs(), the sizes of existing
variably-sized SA entries are obtained from sa_lengths. The case where
a variably-sized SA was being replaced neglected to increment the index
into sa_lengths, so subsequent variable-length SAs would be rewritten
with the wrong length. This patch adds the missing increment operation
so all variably-sized SA entries are stored with their correct lengths.
Another problem was that index into attr_desc was increased even when
an attribute was removed. If that attribute was not the last attribute,
then the last attribute was lost.