time: Fix use-after-free in getdate

getdate would free the buffer pointed to by the result of its call to
strptime, then reference the same buffer later on -- leading to a
use-after-free.  This commit fixes that.

Reported-by: Martin Coufal <mcoufal@redhat.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>