SSE 4.2 memcmp: don't read beyond var boundary

In the SSE 4.2 SCMemcmpLowercase implementation, there would be a
_mm_load_si128 of a 2 byte array. However, _mm_load_si128 loads
16 bytes, causing it to read beyond the var. I don't think this lead
to crashes, as it was a static var, but clangs ASAN complained about
it.

https://buildbot.suricata-ids.org/builders/inliniac/builds/52