MDEV-31791: Recovery on memory-mapped log occasionally fails

recv_ring::copy_if_needed(): If the record wraps around the
memory-mapped ib_logfile0, do copy it also if len==0
(the record consists only of a header, like FREE_PAGE and INIT_PAGE
records do).

recv_sys_t::parse(): Invoke recv_ring::copy_if_needed() for INIT_PAGE
and FREE_PAGE records, so that if these records wrap around the
memory-mapped ib_logfile0, they will be correctly copied to
recv_sys.pages.

Together with commit 0d175968 (MDEV-31354)
this fixes occasional failures of the test innodb.recovery_memory.

storage/innobase/log/log0recv.cc

@@ -2395,8 +2395,6 @@ struct recv_ring : public recv_buf
   const byte *copy_if_needed(const byte *iv, byte *tmp, recv_ring start,
                              size_t len)
   {
-    if (!len)
-      return ptr;
     const size_t s(*this - start);
     ut_ad(s + len <= srv_page_size);
     if (!log_sys.is_encrypted())
@@ -2751,6 +2749,8 @@ recv_sys_t::parse_mtr_result recv_sys_t::parse(source &l, bool if_exists)
           store_freed_or_init_rec(id, (b & 0x70) == FREE_PAGE);
         if (UNIV_UNLIKELY(rlen != 0))
           goto record_corrupted;
+      copy_if_needed:
+        cl= l.copy_if_needed(iv, decrypt_buf, recs, rlen);
         break;
       case EXTENDED:
         if (UNIV_UNLIKELY(!rlen))
@@ -2782,10 +2782,7 @@ recv_sys_t::parse_mtr_result recv_sys_t::parse(source &l, bool if_exists)
         break;
       case OPTION:
         if (rlen == 5 && *l == OPT_PAGE_CHECKSUM)
-        {
-          cl= l.copy_if_needed(iv, decrypt_buf, recs, rlen);
-          break;
-        }
+          goto copy_if_needed;
         /* fall through */
       case RESERVED:
         continue;