asyncnet, net: call SSL_shutdown only when connection established

This commit prevents "SSL_shutdown while in init" errors from happening.

See https://github.com/openssl/openssl/issues/710#issuecomment-253897666

lib/pure/asyncnet.nim

@@ -713,7 +713,14 @@ proc close*(socket: AsyncSocket) =
     socket.fd.AsyncFD.closeSocket()
   when defineSsl:
     if socket.isSsl:
-      let res = SSL_shutdown(socket.sslHandle)
+      let res =
+        # Don't call SSL_shutdown if the connection has not been fully
+        # established, see:
+        # https://github.com/openssl/openssl/issues/710#issuecomment-253897666
+        if SSL_in_init(socket.sslHandle) == 0:
+          SSL_shutdown(socket.sslHandle)
+        else:
+          0
       SSL_free(socket.sslHandle)
       if res == 0:
         discard