patch for buffindexed method

[Kirill Berezin]

Hie workers.
Please review the following patch for storage/buffindexed/buffindexed.c 
for inn 2.4.3. The patch does the following

-  OV_OFFSET macro added to make possible an access to overview blocks 
with indexes over 206000 and corresponding file offsets over 4Gb.

- in ovparse_part_line() added an explicit type conversion  to read a 
length of an overview buffer larger than 4000000.

kirill.