Avoid unsetting nonexistent variables (#1075)

modules/editor/init.zsh

@@ -326,4 +326,4 @@ else
   print "prezto: editor: invalid key bindings: $key_bindings" >&2
 fi
 
-unset key{,map,bindings}
+unset key{,map,_bindings}

modules/history-substring-search/init.zsh

@@ -54,4 +54,6 @@ if [[ -n "$key_info" ]]; then
     bindkey -M "$keymap" "$key_info[Up]" history-substring-search-up
     bindkey -M "$keymap" "$key_info[Down]" history-substring-search-down
   done
+
+  unset keymap
 fi