GH-44: fix "parameter not set" warning by `set -u`
GH-44: fix "parameter not set" warning by `set -u`

$ zsh -f
zsh% set -u
zsh% source zsh-history-substring-search.zsh
zsh% bindkey '^[[A' history-substring-search-up
zsh% bindkey '^[[B' history-substring-search-down
zsh% so<up>
_history-substring-search-begin:11: _history_substring_search_result: parameter not set

joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles