aacsbr: Initialize e_a[1] to -1.

This triggers lAPrev (e_a[0]) on the first SBR frame to be -1. The spec is
somewhat ambiguous to what this value should be but this increases the accuracy
of al_sbr_e_44_1 and similar streams from 14 bits to 15 bits.

Originally committed as revision 22408 to svn://svn.ffmpeg.org/ffmpeg/trunk
