pure_alloc returns cleared memory

pure_alloc returns cleared memory

* src/alloc.c (pure_alloc): Clear any heap-allocated storage.
This is simpler than auditing all the callers to make sure
they don’t assume pure memory is cleared memory, and the
performance implication is nonexistent except when Emacs
is misconfigured.  Also, add an assertion to catch
caller misuse when pure space is exhausted.