The "fast" mode of turbo base64 seem to go too fast on small strings,
disabling the AVX2 optimisation for small string fixes the problem.
See the test named 02113_base64encode_trailing_bytes.sql for an example
of a failing case (only applies when running on a CPU with the AVX2
instruction set).
Ref. 15e044269d/turbob64.h (L106-L111)
