Fix uint8_t comparison with negative value

Commit 7bccf57383cca60a778d5c543ac80c9f62d89ef2 missed this one:
/src/qemu/ui/vnc-enc-tight.c: In function 'send_sub_rect':
/src/qemu/ui/vnc-enc-tight.c:1527: warning: comparison is always true due to limited range of data type

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
