archive/iso9660: another fix for unaligned reads

Commit 79b2366387 added the field `skip`
to support unaligned reads, but set the `offset` field to a wrong
value.  This resulted in miscalculation of `remaining`, causing
an assertion failure.

The fix is to assign `offset` the correct value, but consider the
`skip` value in the assertion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1067

src/archive/plugins/Iso9660ArchivePlugin.cxx

@@ -221,8 +221,8 @@ public:
 		if (new_offset > size)
 			throw std::runtime_error("Invalid seek offset");
 
+		offset = new_offset;
 		skip = new_offset % ISO_BLOCKSIZE;
-		offset = new_offset - skip;
 		buffer.Clear();
 	}
 };
@@ -260,7 +260,7 @@ Iso9660InputStream::Read(std::unique_lock<Mutex> &,
 	if (r.empty()) {
 		/* the buffer is empty - read more data from the ISO file */
 
-		assert(offset % ISO_BLOCKSIZE == 0);
+		assert((offset - skip) % ISO_BLOCKSIZE == 0);
 
 		const ScopeUnlock unlock(mutex);