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/cms.c b/fuzz/cms.c
index 71d8b69..c4477ef 100644
--- a/fuzz/cms.c
+++ b/fuzz/cms.c
@@ -36,3 +36,7 @@
     BIO_free(in);
     return 0;
 }
+
+void FuzzerCleanup(void)
+{
+}