Fix secondary key max length check in index entry

The secondary key max length check has regressed after secondary key encoding was moved to projector in 4.5.0.

Right now it works as follows:

1. If len(key) > maxIndexEntrySize, the key gets skipped
maxIndexEntrySize = 12546 [(4096 * 3) + 256 + 2]
where 256 = MAX_DOCID_LEN and 2 is reserved for len of docid.

2. But after the check, docid and len(docid) are appended to it. This causes the buffer overrun.
The fix is to check len(key) > maxIndexEntrySize - 256 - 2