lavu/samplefmt: return the size of the allocated samples buffer at the next bump

Make the functions av_samples_fill_arrays, av_samples_alloc, and
avcodec_fill_audio_frame return a buffer size rather than 0 in case of
success. This will be enabled at the next libavutil major bump, in order
to preserve backward compatibility.

Returning the size allows to simplify the code, avoiding a few function
calls.

libavutil/samplefmt.c

@@ -171,7 +171,11 @@ int av_samples_fill_arrays(uint8_t **audio_data, int *linesize,
     if (linesize)
         *linesize = line_size;
 
+#if FF_API_SAMPLES_UTILS_RETURN_ZERO
     return 0;
+#else
+    return buf_size;
+#endif
 }
 
 int av_samples_alloc(uint8_t **audio_data, int *linesize, int nb_channels,
@@ -196,7 +200,11 @@ int av_samples_alloc(uint8_t **audio_data, int *linesize, int nb_channels,
 
     av_samples_set_silence(audio_data, 0, nb_samples, nb_channels, sample_fmt);
 
+#if FF_API_SAMPLES_UTILS_RETURN_ZERO
     return 0;
+#else
+    return size;
+#endif
 }
 
 int av_samples_copy(uint8_t **dst, uint8_t * const *src, int dst_offset,