libutils: cleanups for -fsanitize=integer

* Hashing functions rely on integer overflow behavior.
Mark those functions as safe.

* abort() if a passed in size_t value is greater than
UINT32_MAX. This can occur on 64 bit builds where size_t
is larger than uint32_t.

* Special case the index lookup for an empty sorted vector.
Without the special case, size() == 0, and size()-1 underflows.

Change-Id: I343a14b589fc8f0d221c1998ae5d6f0b9e2781f8