xkbcomp: fix stack overflow when evaluating boolean negation The expression evaluator would go into an infinite recursion when evaluating something like this as a boolean: `!True`. Instead of recursing to just `True` and negating, it recursed to `!True` itself again. Bug inherited from xkbcomp. Caught with the afl fuzzer. Signed-off-by: Ran Benita <ran234@gmail.com>