Fix UB vector usage

Fix UB vector usage

It is UB to
- increment past end.
- decrement end iterator for empty vector.
- use operator[] on end iterator.

Also fixes a buffer overflow for c_simdBestPairAlignment=2.

All found with _LIBCPP_DEBUG=1.

Change-Id: Ib21ca875244673b27748a01373e7fc10252a7c44