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.