dfa: avoid use of uninitialized constraint

2020-09-13  Paul Eggert  <eggert@cs.ucla.edu>

	dfa: avoid use of uninitialized constraint

	* lib/dfa.c (merge_nfa_state): Do not initialize the constraint

	to zero here.

	(dfaoptimize): Do it here instead, via xcalloc.  This prevents the

	use of an uninitialized constraint by later code when ! (flags[i]

	& OPT_QUEUED) means merge_nfa_state was not called to initialize

	the constraint.  Problem found by running 'valgrind src/grep -E

	'(^| )*(a|b)*(c|d)*( |$)' < /dev/null' on Ubuntu 18.04.5 x86-64.

	dfa: assume C99 in reorder_tokens

	* lib/dfa.c (reorder_tokens): Assume C99 and simplify.

  position_set *follows = d->follows;

  idx_t nelem = 0;

  d->constraints[tindex] = 0;

  for (idx_t i = 0; i < follows[tindex].nelem; i++)

    {

      idx_t sindex = follows[tindex].elems[i].index;

  position_set *merged = &merged0;

  alloc_position_set (merged, d->nleaves);

  d->constraints = xnmalloc (d->tindex, sizeof *d->constraints);

  d->constraints = xcalloc (d->tindex, sizeof *d->constraints);

  for (idx_t i = 0; i < d->tindex; i++)

    if (flags[i] & OPT_QUEUED)