bug 762: Fix memory leak in ses_goto.

bug 762: Fix memory leak in ses_goto.

In src/session/task.c, if ses_goto() was going to ask the user to
confirm, it did:

    task->session_task.target.frame = null_or_stracpy(target_frame);

It added the struct task to a memory_list, so the structure was freed
when the message box was closed.  The target frame string was however
never freed.  To fix this leak, add the target frame string to the
memory_list too.

Alternatively, this could have been fixed by making post_yes() and
post_no() free the string.  It is however a bit better to use the
memory_list because msg_box() frees that even if it is unable to
display the message box.