Ristretto version: 0.8.0

Steps to reproduce:

1. run ristretto (do not open a file, just run the command without arguments)
2. no image is shown up
3. pres the Delete key on the keyboard

OR

1. open a image file in ristretto
2. keep deleteting all images with the Delete key until no image is shown
3. press the Delete key once more

Expected: nothing should happen

Actuall: SIGSEGV

More info such as backtrace available at https://bugzilla.redhat.com/show_bug.cgi?id=1377128