WeaselJsonParser_create can return nullptr when allocation fails or the
requested stack size is too small. Previously, passing that nullptr to
WeaselJsonParser_reset or WeaselJsonParser_destroy dereferenced it before
doing any work, causing immediate undefined behavior.
Add an early null check to both functions so they behave like free(nullptr)
(i.e., are a safe no-op). Also add a doctest case covering both a null
returned from create and a literal nullptr.
Closes#41
weaselbot