Generated parser triggers segfault with bison 2.7

[Chris Hillery]

See discussion here: https://groups.google.com/forum/?hl=en&fromgroups=#!topic/zorba-dev/Qk4ISLlaCgo

It appears that (at least on Paul's Mac), after re-generating the parser with bison 2.7, a relatively simple query causes Zorba to dump core. Valgrind shows many invalid memory reads/writes from a DirAttr that is destroyed. With earlier bison versions, there are memory leaks in a suspiciously close line number in the parser, which leads me to believe that bison either fixed or introduced a bug which slightly changes how it memory-manages some objects in a way that we don't work nicely with.