Access data after obtaining the lock not before.

It isn't completely clear that this constitutes a race condition, but it will
always be conservative to access the locked data after getting the lock.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/9579)

crypto/ex_data.c

@@ -49,8 +49,8 @@ static EX_CALLBACKS *get_and_lock(OPENSSL_CTX *ctx, int class_index)
          return NULL;
     }
 
-    ip = &global->ex_data[class_index];
     CRYPTO_THREAD_write_lock(global->ex_data_lock);
+    ip = &global->ex_data[class_index];
     return ip;
 }