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
5735efee15540765315aa8c1a230575e756037f7. 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