wavpackenc: make assert more thorough

It was only validating that normal data wasn't filling the buffer.
However, extra data may be written afterwards.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/wavpackenc.c

@@ -2813,7 +2813,7 @@ static int wavpack_encode_block(WavPackEncodeContext *s,
     block_size = bytestream2_tell_p(&pb);
     AV_WL32(out + 4, block_size - 8);
 
-    av_assert0(put_bits_left(&s->pb) > 0);
+    av_assert0(!bytestream2_get_eof(&pb));
 
     return block_size;
 }