Fix #997 custom cache error

Author: Shougo

autoload/deoplete/custom.vim

@@ -40,13 +40,19 @@ function! deoplete#custom#_update_cache() abort
     let s:cached.source_vars[name] = get(source, 'vars', {})
   endfor
   for [name, vars] in items(custom_buffer.source_vars)
+    if !has_key(s:cached.source_vars, name)
+      let s:cached.source_vars[name] = {}
+    endif
     call extend(s:cached.source_vars[name], vars)
   endfor
   let s:cached.filter = {}
   for [name, vars] in items(s:custom.filter)
     let s:cached.filter[name] = vars
   endfor
   for [name, vars] in items(custom_buffer.filter)
+    if !has_key(s:cached.filter, name)
+      let s:cached.filter[name] = {}
+    endif
     call extend(s:cached.filter[name], vars)
   endfor
 endfunction