Address #1452 - Partial revert some changes from #1391

This doesn't seem to reintroduce the original issue. There was
definitely some interplay with #1431

Doing git bisect landed me at commit
2f0c789 as the first bad commit for
issue #1452.

src/core/ev.c

@@ -281,13 +281,9 @@ void janet_async_start(JanetStream *stream, JanetAsyncMode mode, JanetEVCallback
     janet_assert(!fiber->ev_callback, "double async on fiber");
     if (mode & JANET_ASYNC_LISTEN_READ) {
         stream->read_fiber = fiber;
-    } else {
-        stream->read_fiber = NULL;
     }
     if (mode & JANET_ASYNC_LISTEN_WRITE) {
         stream->write_fiber = fiber;
-    } else {
-        stream->write_fiber = NULL;
     }
     fiber->ev_callback = callback;
     fiber->ev_stream = stream;