bash-completion: localize the prev variable
The prev variable is not localzed in the nmcli Bash completion script, which calls _init_completion.
Even though prev does not appear in the completion script, it should still be localized. This variable may otherwise appear in the user's environment and clobber a user-defined variable of the same name, which is bad.
Localize the prev variable.
Closes #741 (closed)