diracdec: clear slice_params_num_buf on allocation failure

Otherwise it can be non-zero next time decode_lowdelay is called, causing
slice_params_buf not to be allocated, leading to a NULL pointer dereference.

The problem was introduced in commit
dcad4677d637cd2f701917e38361fa96b8c9a418.

Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
