Correct return type for BIO_ptr_ctrl

Fixes #17549
CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from #17636)

(cherry picked from commit 984cc9a)

doc/man3/BIO_ctrl.pod

@@ -16,7 +16,7 @@ BIO_get_info_callback, BIO_set_info_callback, BIO_info_cb
 
  long BIO_ctrl(BIO *bp, int cmd, long larg, void *parg);
  long BIO_callback_ctrl(BIO *b, int cmd, BIO_info_cb *cb);
- char *BIO_ptr_ctrl(BIO *bp, int cmd, long larg);
+ void *BIO_ptr_ctrl(BIO *bp, int cmd, long larg);
  long BIO_int_ctrl(BIO *bp, int cmd, long larg, int iarg);
 
  int BIO_reset(BIO *b);