aacenc: Fix the conditions under which 3GPP pre-echo control is run.

aacenc: Fix the conditions under which 3GPP pre-echo control is run.

According to the 3GPP spec:
"Thus the pre-echo control is inactive for the first short window (but
not all short windows in a short frame) after a start block and for
all frames with a stop window sequence."

Currently, pre-echo control is only run when the current frame is not
a short frame, and the previous frame is not a short frame.

patch by Nathan Caldwell saintdev (at) gmail

Originally committed as revision 25587 to svn://svn.ffmpeg.org/ffmpeg/trunk