#201 zvbi compilation warnings

[Ilya Gordeev]

Building zvbi-0.2.35 by gcc-4.8.2 I'v got this compilation warnings:
src/xds_demux.c:829:29: warning: iteration 24u invokes undefined behavior [-Waggressive-loop-optimizations]
contrib/ntsc-cc.c:1164:24: warning: iteration 25u invokes undefined behavior [-Waggressive-loop-optimizations]
contrib/atsc-cc.c:4076:28: warning: iteration 25u invokes undefined behavior [-Waggressive-loop-optimizations]

Propose patch (in attachment), if I understand that code correctly.