Fix a memory leak in the mem bio

If you use a BIO and set up your own buffer that is not freed, the
memory bio will leak the BIO_BUF_MEM object it allocates.

The trouble is that the BIO_BUF_MEM is allocated and kept around,
but it is not freed if BIO_NOCLOSE is set.

The freeing of BIO_BUF_MEM was fairly confusing, simplify things
so mem_buf_free only frees the memory buffer and free the BIO_BUF_MEM
in mem_free(), where it should be done.

Alse add a test for a leak in the memory bio
Setting a memory buffer caused a leak.

Signed-off-by: Corey Minyard <minyard@acm.org>

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8051)