Patch for 5.7

Workaround for compiler bug
ld.so.1: mysqld: fatal: relocation error: file sql/mysqld:
         symbol OPENSSL_sk_new_null: referenced symbol not found
openssl/safestack.h has lots of pragma weak <function>.
All are tagged as WEAK in generated binaries.

Taking the address of OPENSSL_sk_new_null solves the problem.
We do it in sql/net_serv.cc because this file is used by both
client and server and "embedded" versions of client and server.

Change-Id: I63e1a1bf5ecd29bf7c3f0697295c533c4401af41