Revert e8826ded5fea3593d89da2be5c2d81c522070995.

This reverts commit "BUG/MEDIUM: mux_pt: Make sure we don't have a
conn_stream before freeing.".
mux_pt_io_cb() is only used if we have no associated stream, so we will
never have a cs, so there's no need to check that, and we of course have to
destroy the mux in mux_pt_detach() if we have no associated session, or if
there's an error on the connection.

This should be backported to 2.0 and 1.9.

(cherry picked from commit 9dce2c53a8e49d43b501c3025d41705d302b1df1)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/mux_pt.c b/src/mux_pt.c
index 72b8639..008d2bc 100644
--- a/src/mux_pt.c
+++ b/src/mux_pt.c
@@ -51,10 +51,9 @@
 	struct mux_pt_ctx *ctx = tctx;
 
 	conn_sock_drain(ctx->conn);
-	if (ctx->conn->flags & (CO_FL_ERROR | CO_FL_SOCK_RD_SH | CO_FL_SOCK_WR_SH)) {
-		if (!ctx->cs)
-			mux_pt_destroy(ctx);
-	} else
+	if (ctx->conn->flags & (CO_FL_ERROR | CO_FL_SOCK_RD_SH | CO_FL_SOCK_WR_SH))
+		mux_pt_destroy(ctx);
+	else
 		ctx->conn->xprt->subscribe(ctx->conn, ctx->conn->xprt_ctx, SUB_RETRY_RECV,
 		    &ctx->wait_event);
 
@@ -194,7 +193,7 @@
 	    !(conn->flags & (CO_FL_ERROR | CO_FL_SOCK_RD_SH | CO_FL_SOCK_WR_SH))) {
 		ctx->cs = NULL;
 		conn->xprt->subscribe(conn, conn->xprt_ctx, SUB_RETRY_RECV, &ctx->wait_event);
-	} else if (!ctx->cs)
+	} else
 		/* There's no session attached to that connection, destroy it */
 		mux_pt_destroy(ctx);
 }