erts: Fix windows spin bug when stdio is closed
erts: Fix windows spin bug when stdio is closed

Example symptom:
os:cmd("escript something") would cause 100% cpu and even increasing
memory consumption.

Problem:
threaded_reader() keeps calling ReadFile and getting 0 bytes read
which are even sent as {Port, {data, <<>>}} messages from port.

Fix:
Exit threaded_reader() if 0 bytes is read. This was already done but only
if CRLF->LF conversion was disabled, which I don't understand why. Could be
someone was "unlucky" thinking "the CRLF conversion may decrease
bytesTransferred down to zero", which it cannot.

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