crypt-burn: Add sanity check for buffer length

This value is passed to chunk_alloc(), which LGTM complains about.

scripts/crypt_burn.c

@@ -196,6 +196,11 @@ int main(int argc, char *argv[])
 	if (argc > 2)
 	{
 		len = atoi(argv[2]);
+		if (len > (2^30))
+		{
+			fprintf(stderr, "buffer too large (1 GiB limit)\n");
+			return 1;
+		}
 	}
 	if (argc > 3)
 	{