blobfs: add assert in fs_open_blob_create_cb to satisfy static checker

	struct spdk_filesystem *fs = args->fs;

	if (file == NULL) {

		/*

		 * This is from an open with CREATE flag - the file

		 *  is now created so look it up in the file list for this

		 *  filesystem.

		 */

		file = fs_find_file(fs, args->op.open.name);

		assert(file != NULL);

		args->file = file;

	}