Another clang analyzer complaint wrt HT_GENERATE

We're calling mallocfn() and reallocfn() in the HT_GENERATE macro
with the result of a product.  But that makes any sane analyzer
worry about overflow.

This patch keeps HT_GENERATE having its old semantics, since we
aren't the only project using ht.h.  Instead, define a HT_GENERATE2
that takes a reallocarrayfn.