Add a couple missing ATTACHPTR->tree frees.

Add a couple missing ATTACHPTR->tree frees.

While working on ticket 3800, I noticed the tree wasn't being freed
inside OP_COMPOSE_EDIT_HEADERS.  Add a free there, and in the cleanup
at the end of mutt_compose_menu().

A few other sections in mutt_compose_menu() are performing a
free-on-error where the tree isn't allocated yet, so skip it for
those.

I believe this is actually not fixing a memory leak: all attachments
are always at level 0 in the compose menu (as far as I know); so
nothing is ever allocated in the tree pointer.  However since other
parts of the code in compose.c clean this up, it make sense to add
them here too.