fix(frontends/lean/parser): uninitialized var error reported by valgrind

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(tests/lua/threads): increase timeout to make sure it passes the test on slow machines

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/parser): memory leak due to g++ bug

g++ implementation of std::initializer_list has bug.
http://gcc.gnu.org/ml/gcc-bugs/2013-06/msg00095.html

This commit memory leaks triggered by this bug.
It also adds minimal tests to expose three different instances of the problem.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>