slideshow.c: Fix segfault after encountering multiple invalid images in a row
slideshow.c: Fix segfault after encountering multiple invalid images in a row

BlackLight/feh,Caellian/feh,BlackLight/feh,yusiwen/feh,yusiwen/feh,Caellian/feh,BlackLight/feh,Caellian/feh,yusiwen/feh,Caellian/feh