ac3: validate end in ff_ac3_bit_alloc_calc_mask

This fixes an invalid read if end is 0:
     band_end   = ff_ac3_bin_to_band_tab[end-1] + 1;

Depending on what is before the array, this can cause stack smashing,
when band_end becomes too large.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>