CEC: do not join NULL thread

'hdmicec_close' may be called after exit_fd initialized,
but before ctx->thread created.

This patch checks it for NULL before join.

Test: manual
Change-Id: I1c57be56f2cd9956044d9a48fea914b1c1fd0132
Signed-off-by: Oleg Lyovin <ovlevin@salutedevices.com>

cec/hdmi_cec.c

@@ -462,6 +462,7 @@ static int hdmicec_close(struct hdmi_cec_device *dev)
 
     if (ctx->exit_fd > 0) {
         write(ctx->exit_fd, &tmp, sizeof(tmp));
+        if (ctx->thread)
             pthread_join(ctx->thread, NULL);
     }