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.

src/bin/pg_dump/pg_backup_tar.c

@@ -1082,11 +1082,13 @@ _tarAddFile(ArchiveHandle *AH, TAR_MEMBER *th)
 	/*
 	 * Find file len & go back to start.
 	 */
-	fseeko(tmp, 0, SEEK_END);
+	if (fseeko(tmp, 0, SEEK_END) != 0)
+		fatal("error during file seek: %m");
 	th->fileLen = ftello(tmp);
 	if (th->fileLen < 0)
 		fatal("could not determine seek position in archive file: %m");
-	fseeko(tmp, 0, SEEK_SET);
+	if (fseeko(tmp, 0, SEEK_SET) != 0)
+		fatal("error during file seek: %m");
 
 	_tarWriteHeader(th);