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

src/backend/utils/mb/mbutils.c

@@ -1057,11 +1057,16 @@ GetMessageEncoding(void)
 WCHAR *
 pgwin32_message_to_UTF16(const char *str, int len, int *utf16len)
 {
+	int			msgenc = GetMessageEncoding();
 	WCHAR	   *utf16;
 	int			dstlen;
 	UINT		codepage;
 
-	codepage = pg_enc2name_tbl[GetMessageEncoding()].codepage;
+	if (msgenc == PG_SQL_ASCII)
+		/* No conversion is possible, and SQL_ASCII is never utf16. */
+		return NULL;
+
+	codepage = pg_enc2name_tbl[msgenc].codepage;
 
 	/*
 	 * Use MultiByteToWideChar directly if there is a corresponding codepage,
@@ -1086,7 +1091,7 @@ pgwin32_message_to_UTF16(const char *str, int len, int *utf16len)
 		{
 			utf8 = (char *) pg_do_encoding_conversion((unsigned char *) str,
 													  len,
-													  GetMessageEncoding(),
+													  msgenc,
 													  PG_UTF8);
 			if (utf8 != str)
 				len = strlen(utf8);