fix: ucl_expand_single_variable doesn't call free

[andoriyu]

ucl_expand_single_variable doesn't free dst even if need_free set to true and that leaks memory.

[vstakhov]

Good catch, thank you!

[andoriyu]

@vstakhov there still something not right with that code path. I a few tests that interact with variables: one with static substitution and other with var_handler.

When all tests executed one by one on the same thread last test has wrong value in the object.

If I under correctly when I add chunk to parser, I don't have to keep memory alive because parser will copy it. Same for for variables. But my resulting object endup having a few bytes either from another test key or another test value - depends on the order tests are executed.