fix(feh): fix "nounset" error

In the current master, an error message is printed on the TAB
completion with the following command line (where the cursor position
is placed before the marker "[TAB]"):

$ set -u
$ feh --font x[TAB] --fontpath
bash: words[i + 1]: unbound variable

Author: akinomyoga

completions/feh

@@ -30,7 +30,7 @@ _comp_cmd_feh()
             local font_path
             # font_path="$(imlib2-config --prefix 2>/dev/null)/share/imlib2/data/fonts"
             # _comp_compgen -C "$font_path" -- -f -X "!*.@([tT][tT][fF])" -S /
-            for ((i = ${#words[@]} - 1; i > 0; i--)); do
+            for ((i = ${#words[@]} - 2; i > 0; i--)); do
                 if [[ ${words[i]} == -@(C|-fontpath) ]]; then
                     font_path="${words[i + 1]}"
                     _comp_compgen -aC "$font_path" -- \