MDEV-15587 AES test fails, segfaults in EVP_CipherInit_ex

When HAVE_YASSL is defined (due to cmake -DWITH_SSL=bundled
or otherwise), mysys_ssl/my_crypt.cc will #include "yassl.cc"
from the same directory.

When MariaDB 10.2 or later is compiled with GCC 8 and optimizations
are enabled, then the check
  if (iv)
in EVP_CipherInit_ex() can be wrongly optimized away.
The reason appears to be that __attribute__((nonnull)) is attached
to the variable iv, because there is a (no-op) call
memcpy(oiv, iv, ivlen=0) earlier in the code path.

It is possible that this started failing after the code was
refactored in MDEV-10332 (MariaDB 10.2.6). In MariaDB 10.1,
there is a similar memcpy() call in MyCTX_nopad::init(),
but the code appears to work fine.

mysys_ssl/my_crypt.cc

@@ -1,6 +1,6 @@
 /*
  Copyright (c) 2014 Google Inc.
- Copyright (c) 2014, 2017 MariaDB Corporation
+ Copyright (c) 2014, 2019, MariaDB Corporation.
 
  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
@@ -93,7 +93,8 @@ class MyCTX_nopad : public MyCTX
     this->key= key;
     this->klen= klen;
     this->buf_len= 0;
-    memcpy(oiv, iv, ivlen);
+    if (ivlen)
+      memcpy(oiv, iv, ivlen);
     DBUG_ASSERT(ivlen == 0 || ivlen == sizeof(oiv));
 
     int res= MyCTX::init(cipher, encrypt, key, klen, iv, ivlen);