pgsql: Check for fseeko() failure in pg_dump's _tarAddFile().

Check for fseeko() failure in pg_dump's _tarAddFile().

Coverity pointed out, not unreasonably, that we checked fseeko's
result at every other call site but these.  Failure to seek in the
temp file (note this is NOT pg_dump's output file) seems quite
unlikely, and even if it did happen the file length cross-check
further down would probably detect the problem.  Still, that's a
poor excuse for not checking the result of a system call.

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/418414daaa4fc4e88f84c6c456bd0ac1c4c86efd

Modified Files
--------------
src/bin/pg_dump/pg_backup_tar.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)