sandbox: Silence coverity warning in state_read_file()

In this case the value seems save to pass to os_free(). Add a comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Coverity (CID: 165109)

arch/sandbox/cpu/state.c

@@ -78,6 +78,10 @@ static int state_read_file(struct sandbox_state *state, const char *fname)
 err_read:
 	os_close(fd);
 err_open:
+	/*
+	 * tainted scalar, since size is obtained from the file. But we can rely
+	 * on os_malloc() to handle invalid values.
+	 */
 	os_free(state->state_fdt);
 	state->state_fdt = NULL;