Fix segfault reading empty streams

The check_clock_gate() was trying to access to the first event in all
streams. However, streams that are empty don't have any event, so they
cause a NULL dereference. Skipping not active streams avoids the
problem.

src/emu/player.c

@@ -73,6 +73,9 @@ check_clock_gate(struct trace *trace)
 
 	struct stream *stream;
 	DL_FOREACH(trace->streams, stream) {
+		if (!stream->active)
+			continue;
+
 		struct ovni_ev *oev = stream_ev(stream);
 		int64_t sclock = stream_evclock(stream, oev);
 

test/emu/ovni/CMakeLists.txt

@@ -17,3 +17,4 @@ test_emu(no-cpus.c SHOULD_FAIL REGEX "loom .* has no physical CPUs")
 test_emu(sort-cpus-by-loom.c MP)
 test_emu(sort-cpus-by-rank.c MP)
 test_emu(tracedir-subdir.c MP DRIVER "tracedir-subdir.driver.sh")
+test_emu(empty-stream.c SHOULD_FAIL REGEX "model_ovni_finish: thread .* is not dead")

test/emu/ovni/empty-stream.c

@@ -0,0 +1,19 @@
+/* Copyright (c) 2023 Barcelona Supercomputing Center (BSC)
+ * SPDX-License-Identifier: GPL-3.0-or-later */
+
+#include <ovni.h>
+#include "instr.h"
+
+/* Create a trace where a stream doesn't contain any event */
+
+int
+main(void)
+{
+	instr_start(0, 1);
+
+	/* Don't flush the thread to create an empty stream */
+
+	ovni_proc_fini();
+
+	return 0;
+}