lib-storage: Fix searching when search query has invalid keywords.

For example "SEARCH KEYWORD ]" was returning all mails instead of nothing.

src/lib-storage/index/index-search.c

@@ -158,6 +158,11 @@ static int search_arg_match_keywords(struct index_search_context *ctx,
 	const unsigned int *keyword_indexes;
 	unsigned int i, j, count;
 
+	if (search_kws->count == 0) {
+		/* invalid keyword - never matches */
+		return 0;
+	}
+
 	t_array_init(&keyword_indexes_arr, 128);
 	mail_index_lookup_keywords(ctx->view, ctx->mail_ctx.seq,
 				   &keyword_indexes_arr);

src/lib-storage/mail-search.h

@@ -109,6 +109,8 @@ struct mail_search_arg {
 	/* set by mail_search_args_init(): */
 	struct {
 		struct mail_search_args *search_args;
+		/* Note that initialized keywords may be empty if the keyword
+		   wasn't valid in this mailbox. */
 		struct mail_keywords *keywords;
 		struct imap_match_glob *mailbox_glob;
 	} initialized;