parser: don't leak AST nodes for discarded symbols

parser: don't leak AST nodes for discarded symbols

If the parser has symbols on the stack, and then enters an error, it
discards the symbols and fails. But their actions which allocate AST
nodes had already ran. So we must free these to avoid leaks.

We use %destructor declarations, see
http://www.gnu.org/software/bison/manual/html_node/Destructor-Decl.html

Note: byacc only supports %destructor when compiled with
--enable-btyacc. Also, it doesn't support using the parse-param in the
destructor. So we might revert this commit before the next release, or
forget about byacc.

https://github.com/xkbcommon/libxkbcommon/issues/8

Signed-off-by: Ran Benita <ran234@gmail.com>