Add a FuzzerClean() function

This allows to free everything we allocated, so we can detect memory
leaks.

Reviewed-by: Rich Salz <rsalz@openssl.org>
GH: #2023
diff --git a/fuzz/ct.c b/fuzz/ct.c
index 636b195..47b0fc3 100644
--- a/fuzz/ct.c
+++ b/fuzz/ct.c
@@ -40,3 +40,7 @@
     }
     return 0;
 }
+
+void FuzzerCleanup(void)
+{
+}