lavf/dvenc: replace av_fifo_peek2() with av_fifo_generic_peek_at()

This is the only remaining caller of av_fifo_peek2(), which will be
deprecated.

libavformat/dvenc.c

@@ -201,8 +201,9 @@ static void dv_inject_audio(DVMuxContext *c, int channel, uint8_t* frame_ptr)
                 if (of*2 >= size)
                     continue;
 
-                frame_ptr[d]   = *av_fifo_peek2(c->audio_data[channel], of*2+1); // FIXME: maybe we have to admit
-                frame_ptr[d+1] = *av_fifo_peek2(c->audio_data[channel], of*2);   //        that DV is a big-endian PCM
+                // FIXME: maybe we have to admit that DV is a big-endian PCM
+                av_fifo_generic_peek_at(c->audio_data[channel], frame_ptr + d, of * 2, 2, NULL);
+                FFSWAP(uint8_t, frame_ptr[d], frame_ptr[d + 1]);
             }
             frame_ptr += 16 * 80; /* 15 Video DIFs + 1 Audio DIF */
         }