ac3enc: fix bug in stereo rematrixing decision.

The rematrixing strategy reuse flags are not reset between frames, so they
need to be initialized for all blocks, not just block 0.

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 5b54d4b376fbd2c291b8a6dba7d425e28f654bcf)
