mpeg4videodec: silence ubsan warning

s->ac_val[0][0] is of type 'int16_t [16]', but points into a larger buffer.
Here it is used as base pointer to find the correct position in the
larger buffer by adding 's->block_index[n] * 16' and thus as 'int16_t *'.

This fixes clang's ubsan runtime error: index out of bounds for type
'int16_t [16]'

Fixes: test_case.mp4

Found-by: Tyson Smith <twsmith@mozilla.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
