MDEV-19604 WolfSSL breaks binlog_encryption.binlog_incident

Log_event_writer::encrypt_and_write() can pass NULL pointer as source buffer
for the encryption. WolfSSL EVP_CipherUpdate(), rightfully rejects this
as invalid parameter.

Fix  Log_event_writer::encrypt_and_write() and check, with assertion,
that src parameterm is sane in MyCTX::update()

mysys_ssl/my_crypt.cc

@@ -60,6 +60,7 @@ class MyCTX
  }
  virtual int update(const uchar *src, uint slen, uchar *dst, uint *dlen)
  {
+ DBUG_ASSERT(src);
  if (EVP_CipherUpdate(ctx, dst, (int*)dlen, src, slen) != 1)
  return MY_AES_OPENSSL_ERROR;
  return MY_AES_OK;

sql/log_event.cc

@@ -1628,8 +1628,11 @@ int Log_event_writer::encrypt_and_write(const uchar *pos, size_t len)
  return 1;
 
  uint dstlen;
- if (encryption_ctx_update(ctx, pos, (uint)len, dst, &dstlen))
+ if (len == 0)
+ dstlen= 0;
+ else if (encryption_ctx_update(ctx, pos, (uint)len, dst, &dstlen))
  goto err;
+
  if (maybe_write_event_len(dst, dstlen))
  return 1;
  pos= dst;