bug#18361: New 'sort' implementation can crash Emacs

[Paul Eggert]

The new implementation of 'sort' in the trunk invokes qsort (or 
qsort_r), but these functions have undefined behavior if the comparison 
function is ill-behaved.  Since the comparison predicate is 
user-defined, this means a bad user-supplied comparison function could 
crash Emacs.

One possible fix would be to build on the proposed patch in Bug#18360, 
except to change Emacs to always define its own qsort_r substitute, one 
that is known to produce some permutation of the input without crashing 
even if the comparison function is ill-behaved.