aacdec: Fix out of array writes (stack).

Set the element to channel vector (e2c_vec) size to be the maximum
number of aac channel elements. This makes it slightly larger than it
needs to be because CCEs are never mapped to output channel locations.

Also add a check that all input tags (legal or not) will fit.

Split from FFmpeg commit a8d67efa

Signed-off-by: Alex Converse <alex.converse@gmail.com>