pgsql: Focus on ftello result < 0 instead of errno

Focus on ftello result < 0 instead of errno

Rather than reset errno (or just hope that its cleared already),
check just the result of the ftello for < 0 to determine if there
was an issue.

Oversight by me, pointed out by Tom.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5e8e794e3be9fbeddf6f2e2c0515dd0f04c784ec

Modified Files
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src/bin/pg_dump/pg_backup_custom.c |    5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)