doc/examples/resampling_audio: use av_freep() for saftey

also its better in examples to use the safer functions.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/examples/resampling_audio.c

@@ -168,7 +168,7 @@ int main(int argc, char **argv)
         dst_nb_samples = av_rescale_rnd(swr_get_delay(swr_ctx, src_rate) +
                                         src_nb_samples, dst_rate, src_rate, AV_ROUND_UP);
         if (dst_nb_samples > max_dst_nb_samples) {
-            av_free(dst_data[0]);
+            av_freep(&dst_data[0]);
             ret = av_samples_alloc(dst_data, &dst_linesize, dst_nb_channels,
                                    dst_nb_samples, dst_sample_fmt, 1);
             if (ret < 0)