Initialize a variable in all cases

Fix a warning with some GCCs:
/src/qemu/ui/vnc-enc-tight.c: In function `send_sub_rect_nojpeg':
/src/qemu/ui/vnc-enc-tight.c:1458: warning: `ret' might be used uninitialized in this function

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