Reset decodePos in lowlevel decode.

FFmpeg was smart enough to do this for us, but not when we loaded a
savestate.  This is because without a savestate, we returned an initial
small header packet of this size, which gave FFmpeg the idea that seeking
there might help.

But, since we're forcing the decode at a certain position the better thing
is to seek ourselves.

Core/HLE/sceAtrac.cpp

@@ -1781,6 +1781,9 @@ int sceAtracLowLevelDecode(int atracID, u32 sourceAddr, u32 sourceBytesConsumedA
 		u32 sourcebytes = atrac->first.writableBytes;
 		if (sourcebytes > 0) {
 			Memory::Memcpy(atrac->data_buf + atrac->first.size, sourceAddr, sourcebytes);
+			if (atrac->decodePos >= atrac->first.size) {
+				atrac->decodePos = atrac->first.size;
+			}
 			atrac->first.size += sourcebytes;
 		}