Don't reset the packet pointer in ssl3_setup_read_buffer

Don't reset the packet pointer in ssl3_setup_read_buffer

Sometimes this function gets called when the buffers have already been
set up. If there is already a partial packet in the read buffer then the
packet pointer will be set to an incorrect value. The packet pointer already
gets reset to the correct value when we first read a packet anyway, so we
don't also need to do it in ssl3_setup_read_buffer.

Fixes #13729

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16098)