MDEV-32746 SIGSEGV on recovery when using innodb_encrypt_log and PMEM

recv_ring::copy_if_needed(): If len==0, do not decrypt anything,
to avoid unnecessary work and a possible crash.

This is similar to the condition recv_buf::copy_if_needed() where
a similar condition was present since the beginning
(commit 685d958e).

Tested by: Matthias Leich

storage/innobase/log/log0recv.cc

@@ -2409,7 +2409,7 @@ struct recv_ring : public recv_buf
   {
     const size_t s(*this - start);
     ut_ad(s + len <= srv_page_size);
-    if (!log_sys.is_encrypted())
+    if (!len || !log_sys.is_encrypted())
     {
       if (start.ptr + s == ptr && ptr + len <= end())
         return ptr;