never iterate on the edge_vector()

This fixes #552, reported by Rüdiger and Ayrat.

* tests/sanity/style.test: Warn aginst iterations on edge_vector.
* spot/parseaut/parseaut.yy, spot/twaalgos/complete.cc,
spot/twaalgos/parity.cc: Iterate over edges(), not edge_vector().
* tests/core/ltlcross.test: Add a test case for #552.
* NEWS: Mention the bug.

tests/sanity/style.test

@@ -298,6 +298,11 @@ for dir in "$TOP/spot" "$TOP/bin" "$TOP/tests"; do
       $GREP 'catch *([^.]' $tmp | $GREP -v 'const.*&' &&
 	diag 'Always capture exceptions by const reference.'
 
+      # iterating over edge_vector() is suspicious, because the
+      # first edge should not be used.
+      $GREP 'for (.*:.*edge_vector()' $tmp &&
+	diag 'Did you mean to iterate over edges()?'
+
       case $file in
 	*.hh | *.hxx)
 	  if $GREP -E '(<<|>>)' $tmp >/dev/null; then