pgsql: pg_waldump: Fix error message for WAL files smaller than XLOG_BL

pg_waldump: Fix error message for WAL files smaller than XLOG_BLCKSZ.

When opening a WAL file smaller than XLOG_BLCKSZ (e.g. 0 bytes long) while
determining the wal_segment_size, pg_waldump checked errno, despite errno not
being set by the short read. Resulting in a bogus error message.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20220214.181847.775024684568733277.horikyota.ntt@gmail.com
Backpatch: 11-, the bug was introducedin fc49e24fa

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0b1020a96d8e14ba5c5fe22f825c3093160aa83b

Modified Files
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src/bin/pg_waldump/pg_waldump.c | 13 +++++--------
1 file changed, 5 insertions(+), 8 deletions(-)