Simplify hash computation to prevent pointer being classed as tainted.

mem.c:add_trace_entry() -> isc_hash_function() -> isc_siphash24()

129 for (; in != end; in += 8) {

6. byte_swapping: Performing a byte swapping operation on
in implies that it came from an external source, and is
therefore tainted.

130 uint64_t m = U8TO64_LE(in);

(cherry picked from commit 8c983a7e)