pgsql: pg_upgrade: properly detect file copy failure on Windows

pg_upgrade:  properly detect file copy failure on Windows

Previously, file copy failures were ignored on Windows due to an
incorrect return value check.

Report by Manu Joye

Backpatch through 9.1

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fae58d5bede8b5bf3dd17381e7f6b73c1772577f

Modified Files
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src/bin/pg_upgrade/file.c       |    6 +++++-
src/bin/pg_upgrade/pg_upgrade.h |    2 --
2 files changed, 5 insertions(+), 3 deletions(-)