avcodec/ac3enc_template: Don't free uninitialized pointers on error

avcodec/ac3enc_template: Don't free uninitialized pointers on error

The ac3 encoders (fixed- and floating-point AC-3 as well as the EAC-3
encoder) all allocate an array whose elements are pointers to other
buffers. The array is not zeroed initially so that if an allocation of
one of the subbuffers fails, the other pointers are uninitialized.
This causes problems when cleaning, so zero the array initially.

(Only the fixed-point AC-3 encoder was affected by this, because
the other two don't clean up at all in case of errors during init.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>