Fix a ref count mistake and add a long-overdue tf_util_print_backtrace() that helped me find it.

src/util.js.c

@@ -435,6 +435,13 @@ const char* tf_util_backtrace_string()
 	return tf_util_backtrace_to_string(buffer, count);
 }
 
+void tf_util_print_backtrace()
+{
+	const char* bt = tf_util_backtrace_string();
+	tf_printf("%s\n", bt);
+	tf_free((void*)bt);
+}
+
 #if defined(__ANDROID__)
 typedef struct _android_backtrace_t
 {