MDEV-19617 Assertion `src' failed in MyCTX::update

Apprently, sometimes there will be null pointers with 0 length
passed to the MyCTX::update() function, and  will need to return
a valid buffer.

So weaken the assertion, and use a valid pointer for src if it was NULL.

mysys_ssl/my_crypt.cc

@@ -60,7 +60,16 @@ class MyCTX
  }
  virtual int update(const uchar *src, uint slen, uchar *dst, uint *dlen)
  {
- DBUG_ASSERT(src);
+#ifdef HAVE_WOLFSSL
+ // WolfSSL checks parameters and does not like NULL pointers to be passed to function below.
+ if (!src)
+ {
+ static const uchar dummy[MY_AES_BLOCK_SIZE];
+ DBUG_ASSERT(!slen);
+ src=dummy;
+ }
+#endif
+
  if (EVP_CipherUpdate(ctx, dst, (int*)dlen, src, slen) != 1)
  return MY_AES_OPENSSL_ERROR;
  return MY_AES_OK;