fecdec: Use a fixed buffer pool size of 64K.

fecdec: Use a fixed buffer pool size of 64K.

With the new variable sized FEC slices, it may happen that the
number of bytes per slice of the first FEC group is very small. It is
therefore no longer appropriate to base the size of the buffer pool
on this quantity. It happened to be much too small (< 1000 bytes)
for one aac file which caused the fecdec filter to abort early due
to a full buffer pool.

This patch uses a fixed buffer pool size of 64K for the fecdec filter,
which ought to be enough for everybody.