sort: avoid segfault when using two or more threads

sort: avoid segfault when using two or more threads

This change does not fix the actual bug.  That was done by commit
c9db0ac6, "sort: preallocate merge tree nodes to heap".  The fix
was to store each "node" structure on the heap, not on the stack.
Otherwise, a node from one thread's stack could be used in another
thread after the first thread had expired (via pthread_join).
This bug was very hard to trigger when using spinlocks, but
easier once we began using mutexes.
* NEWS (Bug fixes): Mention it.
For details, see http://debbugs.gnu.org/7597.