sdlog2: don't free the logbuffer after disarm

Only free the logbuffer if you exit sdlog2. After disarming, just reset
the pointers. This way, we don't take any risks doing malloc and free.

The actual "double free" was caused by trying to free the perf counter
inside logbuffer_free.

src/modules/sdlog2/sdlog2.c

@@ -807,8 +807,8 @@ void sdlog2_stop_log()
 	/* free log writer performance counter */
 	perf_free(perf_write);
 
-	/* free log buffer */
-	logbuffer_free(&lb);
+	/* reset the logbuffer */
+	logbuffer_reset(&lb);
 
 	mavlink_and_console_log_info(&mavlink_log_pub, "[blackbox] stopped (%lu drops)", skipped_count);
 
@@ -2136,7 +2136,8 @@ int sdlog2_thread_main(int argc, char *argv[])
 	pthread_mutex_destroy(&logbuffer_mutex);
 	pthread_cond_destroy(&logbuffer_cond);
 
-	free(lb.data);
+	/* free log buffer */
+	logbuffer_free(&lb);
 
 	thread_running = false;