doveadm: Do not crash doveadm-server if input file missing

save commands expects valid input file for it to work,
if we are not running for cli, and input file is not
provided, provide EINVAL error to caller via i_stream_error.

src/doveadm/doveadm-mail.c

@@ -227,6 +227,11 @@ void doveadm_mail_get_input(struct doveadm_mail_cmd_context *ctx)
 	if (ctx->cmd_input != NULL)
 		return;
 
+	if (!ctx->cli && ctx->conn == NULL) {
+		ctx->cmd_input = i_stream_create_error_str(EINVAL, "Input stream missing (provide with file parameter)");
+		return;
+	}
+
 	if (ctx->conn != NULL)
 		inputs[0] = i_stream_create_dot(ctx->conn->input, FALSE);
 	else {