Fix some pg_verifybackup issues reported by Coverity.

Fix some pg_verifybackup issues reported by Coverity.

Commit 8dfd3129027969fdd2d9d294220c867d2efd84aa introduced a few
problems. verify_tar_file() forgot to free a buffer; the leak can't
add up to anything material, but might as well fix it.
precheck_tar_backup_file() intended to return after reporting an
error but didn't actually do so. member_copy_control_data() could
try to copy zero bytes (and maybe Coverity thinks it can even be
trying to copy a negative number of bytes).

Per discussion with Tom Lane.

Discussion: http://postgr.es/m/1240823.1727629418@sss.pgh.pa.us