demmt: fix minor address annotation bug

found by clang's -Wtautological-compare:
/home/mupuf/nvidia/envytools/rnn/demmt_objects.c:887:72: warning: comparison of unsigned expression >= 0 is always true [-Wtautological-compare]
if (buf == nvc0_3d.code.buffer && s->address - nvc0_3d.code.address >= 0)