bash-completion: fix completion of complete verbs

When doing 'command verb<TAB>', the arguments for verb would be
proposed, but it is too early. We should complete verb first.

https://bugs.freedesktop.org/show_bug.cgi?id=74596

shell-completion/bash/systemctl

@@ -146,7 +146,7 @@ _systemctl () {
                   [TARGETS]='set-default'
         )
 
-        for ((i=0; i <= COMP_CWORD; i++)); do
+        for ((i=0; i < COMP_CWORD; i++)); do
                 if __contains_word "${COMP_WORDS[i]}" ${VERBS[*]} &&
                  ! __contains_word "${COMP_WORDS[i-1]}" ${OPTS[ARG]}; then
                         verb=${COMP_WORDS[i]}