posix: Fix some crashes in wordexp [BZ #18096]

Without these fixes, the first three included tests segfault (on a
NULL dereference); the fourth aborts on an assertion, which is itself
unnecessary.

Signed-off-by: Julian Squires <julian@cipht.net>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>