lavf: Do not compute the packet duration based on the bitrate if the frame_size can be determined.

This fixes issues when the bitrate is variable or inaccurate but the
frame size has not been determined yet.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -779,7 +779,7 @@ static int get_audio_frame_size(AVCodecContext *enc, int size)
             frame_size = (size << 3) / (bits_per_sample * enc->channels);
         } else {
             /* used for example by ADPCM codecs */
-            if (enc->bit_rate == 0)
+            if (enc->bit_rate == 0 || determinable_frame_size(enc))
                 return -1;
             frame_size = ((int64_t)size * 8 * enc->sample_rate) / enc->bit_rate;
         }

tests/ref/fate/wtv-demux

 #tb 0: 1/10000000
 #tb 1: 1/10000000
-1,         -2,         -2,   180000,      576, 0x9b6e1638
-1,     179998,     179998,   240000,      576, 0x0ca91183
-1,     419998,     419998,   240000,      576, 0xec6a180f
-1,     659998,     659998,   240000,      576, 0x478a2b9b
-1,     899998,     899998,   240000,      576, 0x00fa15b3
-1,    1139998,    1139998,   240000,      576, 0xfb551816
+1,         -2,         -2,   240000,      576, 0x9b6e1638
+1,     239998,     239998,   240000,      576, 0x0ca91183
+1,     479998,     479998,   240000,      576, 0xec6a180f
+1,     719998,     719998,   240000,      576, 0x478a2b9b
+1,     959998,     959998,   240000,      576, 0x00fa15b3
+1,    1199998,    1199998,   240000,      576, 0xfb551816
 1,    1439998,    1439998,   240000,      576, 0x422e12bd
 1,    1679998,    1679998,   240000,      576, 0xa7581b29
 1,    1919998,    1919998,   240000,      576, 0xd4b31a74