Set the SBR mode to implicit on ADTS frames occurring before the output is locked.

Originally committed as revision 20700 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aac.c

@@ -1687,6 +1687,8 @@ static int parse_adts_frame_header(AACContext *ac, GetBitContext *gb)
         } else if (ac->output_configured != OC_LOCKED) {
             ac->output_configured = OC_NONE;
         }
+        if (ac->output_configured != OC_LOCKED)
+            ac->m4ac.sbr = -1;
         ac->m4ac.sample_rate     = hdr_info.sample_rate;
         ac->m4ac.sampling_index  = hdr_info.sampling_index;
         ac->m4ac.object_type     = hdr_info.object_type;