contrib/plugins/bbv.c: Check if file is NULL

The file pointer can be NULL when e.g., opening the file failed.
vcpu_interval_exec() already implements a NULL-pointer check, but
plugin_exit() misses it. Handle the condition by adding the missing
check to plugin_exit().

Fixes: 0d279bec0f ("contrib/plugins: Add a plugin to generate basic block vectors")
Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260307-bbv-v1-1-d5757d1deac8@rsg.ci.i.u-tokyo.ac.jp
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

contrib/plugins/bbv.c

@@ -30,8 +30,13 @@ static uint64_t interval = 100000000;
 
 static void plugin_exit(qemu_plugin_id_t id, void *p)
 {
+    Vcpu *vcpu;
+
     for (int i = 0; i < qemu_plugin_num_vcpus(); i++) {
-        fclose(((Vcpu *)qemu_plugin_scoreboard_find(vcpus, i))->file);
+        vcpu = qemu_plugin_scoreboard_find(vcpus, i);
+        if (vcpu->file) {
+            fclose(vcpu->file);
+        }
     }
 
     g_hash_table_unref(bbs);