av_get_audio_frame_duration: fix IMC

Fixes Ticket1658
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -2391,7 +2391,6 @@ int av_get_audio_frame_duration(AVCodecContext *avctx, int frame_bytes)
     case AV_CODEC_ID_GSM:
     case AV_CODEC_ID_QCELP:
     case AV_CODEC_ID_RA_288:       return  160;
-    case AV_CODEC_ID_IMC:          return  256;
     case AV_CODEC_ID_AMR_WB:
     case AV_CODEC_ID_GSM_MS:       return  320;
     case AV_CODEC_ID_MP1:          return  384;
@@ -2470,6 +2469,8 @@ int av_get_audio_frame_duration(AVCodecContext *avctx, int frame_bytes)
                 return 6 * frame_bytes / ch;
             case AV_CODEC_ID_PCM_LXF:
                 return 2 * (frame_bytes / (5 * ch));
+            case AV_CODEC_ID_IMC:
+                return 4 * frame_bytes / ch;
             }
 
             if (tag) {