BUILD: ssl: add an unchecked version of __conn_get_ssl_sock_ctx()

BUILD: ssl: add an unchecked version of __conn_get_ssl_sock_ctx()

First gcc, then now coverity report possible null derefs in situations
where we know these cannot happen since we call the functions in
contexts that guarantee the existence of the connection and the method
used. Let's introduce an unchecked version of the function for such
cases, just like we had to do with objt_*. This allows us to remove the
ALREADY_CHECKED() statements (which coverity doesn't see), and addresses
github issues #1643, #1644, #1647.