pgsql: Fail pgwin32_message_to_UTF16() for SQL_ASCII messages.

Fail pgwin32_message_to_UTF16() for SQL_ASCII messages.

The function had been interpreting SQL_ASCII messages as UTF8, throwing
an error when they were invalid UTF8.  The new behavior is consistent
with pg_do_encoding_conversion().  This affects LOG_DESTINATION_STDERR
and LOG_DESTINATION_EVENTLOG, which will send untranslated bytes to
write() and ReportEventA().  On buildfarm member bowerbird, enabling
log_connections caused an error whenever the role name was not valid
UTF8.  Back-patch to 9.4 (all supported versions).

Discussion: https://postgr.es/m/20190512015615.GD1124997@rfd.leadboat.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ff7555c4f13aa0c62f8e1ad890b2a2c0f512b4fc

Modified Files
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src/backend/utils/mb/mbutils.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)