Don't call pgwin32_message_to_UTF16() without CurrentMemoryContext.

PostgreSQL running as a Windows service crashed upon calling
write_stderr() before MemoryContextInit().  This fix completes work
started in 5735efee15.  Messages this
early contain only ASCII bytes; if we removed the CurrentMemoryContext
requirement, the ensuing conversions would have no effect.  Back-patch
to 9.3 (all supported versions).

Takayuki Tsunakawa, reviewed by Michael Paquier.

Discussion: https://postgr.es/m/0A3221C70F24FB45833433255569204D1F80CC73@G01JPEXMBYT05

src/backend/utils/error/elog.c

@@ -2117,10 +2117,15 @@ write_eventlog(int level, const char *line, int len)
 	 * try to convert the message to UTF16 and write it with ReportEventW().
 	 * Fall back on ReportEventA() if conversion failed.
 	 *
+	 * Since we palloc the structure required for conversion, also fall
+	 * through to writing unconverted if we have not yet set up
+	 * CurrentMemoryContext.
+	 *
 	 * Also verify that we are not on our way into error recursion trouble due
 	 * to error messages thrown deep inside pgwin32_message_to_UTF16().
 	 */
 	if (!in_error_recursion_trouble() &&
+		CurrentMemoryContext != NULL &&
 		GetMessageEncoding() != GetACPEncoding())
 	{
 		utf16 = pgwin32_message_to_UTF16(line, len, NULL);