Fix autoreload-nop accidentally selecting on stdin

When sxiv is built with AUTORELOAD=nop if something is feed via stdin it
will start to infinitely call select() leading to a 100% CPU usage.

Thanks to @sdx23 for helping in analyzing that problem!

Fixes issue #297.

autoreload_nop.c

@@ -20,7 +20,7 @@
 
 void arl_init(arl_t *arl)
 {
-	(void) arl;
+	arl->fd = -1;
 }
 
 void arl_cleanup(arl_t *arl)