avcodec/aac_fixed: Fix a bug in spectral_to_sample()

There was fixed number of loops (2048) in preparation for resampler, so
when number of samples is smaller than this, there would be an overflow on
ret_buf.

For some reason this behavior popped out only under valgrind with
--disable-memory-poisoning option.

This is now fixed and number of loops depends on actual number of samples.

Signed-off-by: Nedeljko Babic <nedeljko.babic@rt-rk.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
