BUG/MEDIUM: Make sure we leave the session list in session_free().

BUG/MEDIUM: Make sure we leave the session list in session_free().

In session_free(), if we're about to destroy a connection that had no mux,
make sure we leave the session_list before calling conn_free(). Otherwise,
conn_free() would call session_unown_conn(), which would potentially free
the associated srv_list, but session_free() also frees it, so that would
lead to a double free, and random memory corruption.

This should be backported to 1.9 and 2.0.