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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d02768ddd1791bab742cf131de726d0a36b3f337

Modified Files
--------------
src/backend/utils/mb/mbutils.c        | 9 +++++++--
src/bin/pg_dump/t/010_dump_connstr.pl | 6 ++----
src/bin/scripts/t/200_connstr.pl      | 6 ++----
3 files changed, 11 insertions(+), 10 deletions(-)