vnc-enc-tight: fix off-by-one bug

In tight_encode_indexed_rect32, buf(or src)’s size is count. In for loop,
the logic is supposed to be that i is an index into src, i should be
incremented when incrementing src.

This is broken when src is incremented but i is not before while loop,
resulting in off-by-one bug in while loop.

Signed-off-by: He Rongguang <herongguang.he@huawei.com>
Message-id: 5784B8EB.7010008@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
