adpcm: Replace any of the leaf nodes in the heap

By not looking for the exactly largest node, we avoid an O(n) seek through
the leaf nodes. Just pick one (not the same one every time) and try replacing
that node with the new one.

For -trellis 8, this lowers the run time from 190 to 158 seconds,
for a 30 second 44 kHz mono sample, on my machine.

Originally committed as revision 25733 to svn://svn.ffmpeg.org/ffmpeg/trunk